Laser-assisted method for parting crystalline material

ABSTRACT

A crystalline material processing method includes forming subsurface laser damage at a first average depth position to form cracks in the substrate interior propagating outward from at least one subsurface laser damage pattern, followed by imaging the substrate top surface, analyzing the image to identify a condition indicative of presence of uncracked regions within the substrate, and taking one or more actions responsive to the analyzing. One potential action includes changing an instruction set for producing subsequent laser damage formation (at second or subsequent average depth positions), without necessarily forming additional damage at the first depth position. Another potential action includes forming additional subsurface laser damage at the first depth position. The substrate surface is illuminated with a diffuse light source arranged perpendicular to a primary substrate flat and positioned to a first side of the substrate, and imaged with an imaging device positioned to an opposing second side of the substrate.

STATEMENT OF RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 16/274,064 filed on Feb. 12, 2019, which claims priority toU.S. Provisional Patent Application No. 62/786,333 filed on Dec. 29,2018 and to U.S. Provisional Patent Application No. 62/803,340 filed onFeb. 8, 2019, wherein the entire disclosures of the foregoingapplications are hereby incorporated by reference herein. Thisapplication also incorporated by reference the entire disclosure of U.S.patent application Ser. No. 16/274,045 filed on Feb. 12, 2019.

TECHNICAL FIELD

The present disclosure relates to methods for processing crystallinematerials, and more specifically to laser-assisted methods for partingor removing relatively thin layers of crystalline material from asubstrate, such as a boule or a wafer.

BACKGROUND

Various microelectronic, optoelectronic, and microfabricationapplications require thin layers of crystalline materials as a startingstructure for fabricating various useful systems. Traditional methodsfor cutting thin layers (e.g., wafers) from large diameter crystallineingots of crystalline materials have involved use of wire saws. Wiresawing technology has been applied to various crystalline materials,such as silicon, sapphire, and silicon carbide. A wire saw tool includesan ultra-fine steel wire (typically having a diameter of 0.2 mm or less)that is passed through grooves of one or many guide rollers. Two slicingmethods exist, namely, loose abrasive slicing and fixed abrasiveslicing. Loose abrasive slicing involves application of a slurry(typically a suspension of abrasives in oil) to a steel wire running athigh speed, whereby the rolling motion of abrasives between the wire andthe workpiece results in cutting of an ingot. Unfortunately, theenvironmental impact of slurry is considerable. To reduce such impact, awire fixed with diamond abrasives may be used in a fixed abrasiveslicing method that requires only a water-soluble coolant liquid (not aslurry). High-efficiency parallel slicing permits a large number ofwafers to be produced in a single slicing procedure. FIG. 1 illustratesa conventional wire saw tool 1 including parallel wire sections 3extending between rollers 4A-4C and arranged to simultaneously saw aningot 2 into multiple thin sections (e.g., wafers 8A-8G) each having aface generally parallel to an end face 6 of the ingot 2. During thesawing process, the wire sections 3 supported by the rollers 4A-4C maybe pressed in a downward direction 5 toward a holder 7 underlying theingot 2. If the end face 6 is parallel to a crystallographic c-plane ofthe ingot 2, and the wire sections 3 saw through the ingot 2 parallel tothe end face 6, then each resulting wafer 8A-8G will have an “on-axis”end face 6′ that is parallel to the crystallographic c-plane.

It is also possible to produce vicinal (also known as offcut or“off-axis”) wafers having end faces that are not parallel to thecrystallographic c-plane. Vicinal wafers (e.g., of SiC) having a 4degree offcut are frequently employed as growth substrates forhigh-quality physical vapor transport and epitaxial growth of othermaterials (e.g., AlN and other Group III nitrides). Vicinal wafers maybe produced either by growing an ingot in a direction away from thec-axis (e.g., growing over a vicinal seed material) and sawing the ingotperpendicular to the ingot sidewalls), or by growing an ingot startingwith an on-axis seed material and sawing the ingot at an angle to thatdeparts from perpendicular to the ingot sidewalls.

Wire sawing of semiconductor materials involves various limitations.Kerf losses based on the width of material removed per cut are inherentto saw cutting, and represent a significant loss of semiconductormaterial. Wire saw cutting applies moderately high stress to wafers,resulting in non-zero bow and warp characteristics. Processing times fora single boule (or ingot) are very long, and events like wire breaks canincrease processing times and lead to undesirable loss of material.Wafer strength may be reduced by chipping and cracking on the cutsurface of a wafer. At the end of a wire sawing process, the resultingwafers must be cleaned of debris.

In the case of silicon carbide (SiC) having high wear resistance (and ahardness comparable to diamond and boron nitride), wire sawing mayrequire significant time and resources, thereby entailing significantproduction costs. SiC substrates enable fabrication of desirable powerelectronic, radio frequency, and optoelectronic devices. SiC occurs inmany different crystal structures called polytypes, with certainpolytypes (e.g., 4H—SiC and 6H—SIC) having a hexagonal crystalstructure.

FIG. 2 is a first perspective view crystal plane diagram showing thecoordinate system for a hexagonal crystal such as 4H—SiC, in which thec-plane ((0001) plane, corresponding to a [0001] (vertical) direction ofcrystal growth) is perpendicular to both the m-plane ((1100) plane) andthe a-plane ((1120) plane), with the (1100) plane being perpendicular tothe [1100] direction, and the (1120) plane being perpendicular to the[11{umlaut over (2)}0] direction. FIG. 3 is a second perspective viewcrystal plane diagram for a hexagonal crystal, illustrating a vicinalplane 9 that is non-parallel to the c-plane, wherein a vector 10 (whichis normal to the vicinal plane 9) is tilted away from the [0001]direction by a tilt angle β, with the tilt angle β being inclined(slightly) toward the [1120] direction. FIG. 4A is a perspective viewwafer orientation diagram showing orientation of a vicinal wafer 11Arelative to the c-plane ((0001) plane), in which a vector 10A (which isnormal to the wafer face 9A) is tilted away from the [0001] direction bya tilt angle β. This tilt angle β is equal to an orthogonal tilt (ormisorientation angle) β that spans between the (0001) plane and aprojection 12A of the wafer face 9A. FIG. 4B is a simplifiedcross-sectional view of the vicinal wafer 11A superimposed over aportion of an ingot 14A (e.g., an on-axis ingot having an end face 6Aparallel to the (0001) plane) from which the vicinal wafer 11A wasdefined. FIG. 4B shows that the wafer face 9A of the vicinal wafer 11Ais misaligned relative to the (0001) plane by a tilt angle β.

FIG. 5 is a top plan view of an exemplary SiC wafer 25 including anupper face 26 (e.g., that is parallel to the (0001) plane (c-plane), andperpendicular to the direction) and laterally bounded by a generallyround edge 27 (having a diameter D) including a primary flat 28 (havinga length L_(F)) that is perpendicular to the (1120) plane, and parallelto the [1120] direction. A SiC wafer may include an outer surface thatis misaligned with (e.g., off-axis at an oblique angle relative to) thec-plane.

Due to difficulties associated with making and processing SiC, SiCdevice wafers have a high cost relative to wafers of various othersemiconductor materials. Typical kerf losses obtained from wire sawingSiC may be approximately 250 microns or more per wafer, which is quitesignificant considering that the wafers resulting from a wire sawingprocess may be roughly 350 microns thick and subsequently thinned (bygrinding) to a final thickness of approximately 100 to 180 micronsdepending on the end use. It has been impractical to slice wafersthinner than about 350 microns considering wire sawing and devicefabrication issues.

To seek to address limitations associated with wire sawing, alternativetechniques for removing thin layers of semiconductor materials from bulkcrystals have been developed. One technique involving removal of a layerof silicon carbide from a larger crystal is described in Kim et al.,“4H—SiC wafer slicing by using femtosecond laser double pulses,” OpticalMaterials Express 2450, vol. 7, no. 7 (2017). Such technique involvesformation of laser-written tracks by impingement of laser pulses onsilicon carbide to induce subsurface damage, followed by adhesion of thecrystal to a locking jig and application of tensile force to effectuatefracture along a subsurface damage zone. Use of the laser to weakenspecific areas in the material followed by fracture between those areasreduces the laser scanning time.

Additional separation techniques involving formation of laser subsurfacedamage with a pulsed laser beam to a SiC ingot and subsequent inducementof fracture by application of ultrasonic vibration are disclosed by U.S.Pat. Nos. 9,925,619 and 10,155,323 to Disco Corporation. Additionaltechniques for removing thin layers of semiconductor materials from bulkcrystals are disclosed in U.S. Patent Application Publication No.2018/0126484A1 to Siltectra GmbH.

Tools for forming laser subsurface damage in semiconductor materials areknown in the art and commercially available from various providers, suchas Disco Corporation (Tokyo, Japan). Such tools permit laser emissionsto be focused within an interior of a crystalline substrate, and enablelateral movement of a laser relative to the substrate. Typical laserdamage patterns include formation of parallel lines that are laterallyspaced relative to one another at a depth within a crystalline materialsubstrate. Parameters such as focusing depth, laser power, translationspeed, etc. may be adjusted to impart laser damage, but adjustment ofcertain factors involves tradeoffs. Increasing laser power tends toimpart greater subsurface damage that may increase ease of fracturing(e.g., by reducing the stress required to complete fracturing), butgreater subsurface damage increases surface irregularities alongsurfaces exposed by fracturing, such that additional processing may berequired to render such surfaces sufficiently smooth for subsequentprocessing (e.g., for incorporation in electronic devices). Reducinglateral spacing between subsurface laser damage lines may also increaseease of fracturing, but a reduction in spacing between laser damagelines increases the number of translational passes between a substrateand a laser, thereby reducing tool throughput. Additionally, resultsobtained by laser processing may vary within a substrate, depending onlateral or radial position at a particular vertical depth, and/ordepending on vertical position of a substrate face relative to itsoriginal growth position as part of an ingot.

Variations in material and/or optical properties within a thicksubstrate such as a SiC ingot, and also among different ingots of thesame composition, render it challenging to easily fabricate wafers ofrepeatably uniform thickness by laser processing and subsequent fracturewhile avoiding unnecessary material loss.

Accordingly, the art continues to seek improved laser-assisted methodsfor parting or removing relatively thin layers of crystalline (e.g.,semiconductor) material from a substrate to address issues associatedwith conventional methods.

SUMMARY

The present disclosure relates in various aspects to methods forprocessing a crystalline material substrate and a material processingapparatus. Imaging and analysis of uncracked regions following formationof subsurface laser damage in a substrate are used as an indicator todetermine when additional laser substrate damage is necessary at a firstdepth position and/or when an instruction set for forming subsurfacelaser damage at subsequent depth positions should be changed, therebyaddressing variation in laser damage formation requirements (e.g., laserpower, laser focusing depth, number of damage formation passes) fromsubstrate to substrate, as well as at different depth positions within asingle substrate. A crystalline material processing method includesgenerating subsurface laser damage sites in areas of the crystallinematerial at a first average depth position to promote formation ofcracks in the substrate interior propagating outward from a subsurfacelaser damage pattern, imaging the substrate top surface, analyzing theimage to identify a condition indicative of presence of uncrackedregions within the substrate, and taking one or more actions responsiveto the analyzing (e.g., upon attainment of appropriate conditions). Onepotential action includes forming supplemental subsurface laser damageat the first average depth position to promote formation of additionalcracks in the uncracked regions, for formation of a first reducedthickness portion of the substrate (e.g., a first wafer). Anotherpotential action includes changing an instruction set for producingsubsequent subsurface laser damage formation (at second or subsequentaverage depth positions, for formation of second and any reducedthickness portions of the substrate), without necessarily formingadditional damage at the first average depth position. The laser damagefacilitates subsequent fracture of the substrate to yield multiplesubstrate portions of reduced thickness. A material processing apparatusincludes a laser processing station having a laser, at least onetranslation stage, a diffuse light source arranged to be positioned to afirst lateral side of a substrate, and an imaging device positioned toan opposing second lateral side of the substrate. The light source maybe positioned substantially perpendicular to a primary flat of thesubstrate and/or within ±5 degrees of perpendicular to a <1120>direction of a hexagonal crystal structure of the substrate, to enhancevisibility of uncracked regions through the top surface of thesubstrate.

In one aspect, the disclosure relates to a crystalline materialprocessing method comprising: supplying emissions of a laser focusedalong a first average depth position within an interior of a crystallinematerial of a substrate, and effecting relative lateral movement betweenthe laser and the substrate, to form subsurface laser damage having atleast one subsurface laser damage pattern, wherein the at least onesubsurface laser damage pattern is configured to promote formation of atleast one plurality of cracks in the interior of the substratepropagating outward from substantially the at least one subsurface laserdamage pattern; following formation of the at least one subsurface laserdamage pattern, generating at least one image of a top surface of thesubstrate; analyzing the at least one image to identify a conditionindicative of presence of uncracked regions in the interior of thesubstrate; and responsive to the analyzing, performing at least one ofthe following steps (i) or (ii): (i) effecting relative movement betweenthe laser and the substrate while supplying emissions of the laserfocused within the interior of the substrate in at least the uncrackedregions to form supplemental subsurface laser damage to supplement theat least one subsurface laser damage pattern and promote formation ofadditional cracks in the uncracked regions along or proximate to thefirst average depth position, for formation of a first reduced thicknessportion of the substrate; or (ii) changing an instruction set,associated with the substrate, for forming subsurface laser damage whenproducing subsurface laser damage patterns at a second average depthposition and any subsequent average depth positions in the substrate,for formation of at least one additional reduced thickness portion ofthe substrate.

In certain embodiments, the analyzing comprises quantifying a top areaproperty of the one or more uncracked regions in the interior of thesubstrate, and comparing the top area property to at least onepredetermined threshold area property.

In certain embodiments, the at least one predetermined threshold areaproperty comprises a first predetermined threshold area property and asecond predetermined threshold area property, wherein the secondpredetermined threshold area property is greater than the firstpredetermined threshold area property, and the method comprises:performing step (ii) if the top area property is at least as large asthe first predetermined threshold area property; and performing step (i)if the top area property is at least as large as the secondpredetermined threshold area property.

In certain embodiments, the method comprises performing both of steps(i) and (ii) responsive to the analyzing.

In certain embodiments, step (ii) comprises adjusting at least one of(a) average laser power, (b) laser focusing depth relative to an exposedsurface of the substrate, or (c) number of laser damage formationpasses, when producing subsurface laser damage patterns at the secondaverage depth position and any subsequent average depth positions in thesubstrate.

In certain embodiments, the changing of the instruction set according tostep (ii) comprises increasing average laser power by a value in a rangeof from 0.15 to 0.35 watts.

In certain embodiments, step (i) comprises adjusting at least one of (a)average laser power, or (b) laser focusing depth relative to an exposedsurface of the substrate, when producing the supplemental subsurfacelaser damage to supplement the at least one subsurface laser damagepattern and promote formation of additional cracks in the uncrackedregions along or proximate to the first average depth position.

In certain embodiments, the substrate comprises a generally round edgehaving a primary flat, and the generating of the at least one imagecomprises (a) illuminating the top surface with diffuse light generatedby a diffuse light source arranged to a first lateral side of thesubstrate and arranged substantially perpendicular to the primary flat,and (b) capturing the at least one image with an imaging device arrangedto an opposing second lateral side of the substrate.

In certain embodiments, the crystalline material comprises a hexagonalcrystal structure; and the generating of the at least one imagecomprises (a) illuminating the top surface with diffuse light generatedby a diffuse light source arranged to a first lateral side of thesubstrate and arranged within ±5 degrees of perpendicular to a <1120>direction of the hexagonal crystal structure, and (b) capturing the atleast one image with an imaging device arranged to a second lateral sideof the substrate that opposes the first lateral side.

In certain embodiments, the at least one subsurface laser damage patterncomprises a first subsurface laser damage pattern and a secondsubsurface laser damage pattern that is formed after the firstsubsurface laser damage pattern; the first subsurface laser damagepattern comprises a first plurality of substantially parallel lines andthe second subsurface laser damage pattern; lines of the secondplurality of substantially parallel lines are interspersed among linesof the first plurality of substantially parallel lines; and at leastsome lines of the second plurality of substantially parallel lines donot cross any lines of the first plurality of substantially parallellines.

In certain embodiments, each line of the second plurality ofsubstantially parallel lines is arranged between a different pair ofadjacent lines of the first plurality of substantially parallel lines.

In certain embodiments, each line of the first plurality ofsubstantially parallel lines and each line of the second plurality ofsubstantially parallel lines is within ±5 degrees of perpendicular to a<1120> direction of a hexagonal crystal structure of the crystallinematerial and substantially parallel to a surface of the substrate.

In certain embodiments, the at least one subsurface laser damage patterncomprises a first subsurface laser damage pattern and a secondsubsurface laser damage pattern that is formed after the firstsubsurface laser damage pattern; the at least one plurality ofsubstantially parallel lines comprises a first plurality ofsubstantially parallel lines and a second plurality of substantiallyparallel lines; lines of the first plurality of substantially parallellines are non-parallel to lines of the second plurality of substantiallyparallel lines; an angular direction of lines of the second plurality ofsubstantially parallel lines differs by no more than 10 degrees from anangular direction of lines of the first plurality of substantiallyparallel lines; and at least some lines of the second plurality ofsubstantially parallel lines do not cross any lines of the firstplurality of substantially parallel lines.

In certain embodiments, the at least one subsurface laser damage patternfurther comprises a third subsurface laser damage pattern that is formedafter the second subsurface laser damage pattern; the at least oneplurality of substantially parallel lines further comprises a thirdplurality of substantially parallel lines; the at least one plurality ofcracks comprises first, second, and third pluralities of cracks; thefirst subsurface laser damage pattern forms the first plurality ofcracks in the interior of the substrate propagating laterally outwardfrom lines of the first plurality of substantially parallel lines; thesecond subsurface laser damage pattern forms the second plurality ofcracks in the interior of the substrate propagating laterally outwardfrom lines of the second plurality of substantially parallel lines, andthe second plurality of cracks is non-connecting with the firstplurality of cracks; and the third subsurface laser damage pattern formsthe third plurality of cracks in the interior of the substratepropagating laterally outward from lines of the third plurality ofsubstantially parallel lines, wherein at least some cracks of the thirdplurality of cracks connect with at least some cracks of the firstplurality of cracks and with at least some cracks of the secondplurality of cracks.

In certain embodiments, the method further comprises detecting acondition indicative of non-uniform doping of the crystalline materialacross at least a portion of a surface of the substrate, the non-uniformdoping including a first doping region and a second doping region; andresponsive to detection of the condition indicative of non-uniformdoping of the crystalline material, performing at least one of thefollowing steps (A) or (B): (A) altering laser power to provide laseremissions at a first power level when forming subsurface laser damage inthe first doping region and provide laser emissions at a second powerlevel when forming subsurface laser damage in the second doping region,during formation of the at least one subsurface laser damage pattern; or(B) changing average depth for formation of subsurface laser damage inthe substrate when forming subsurface laser damage in one of the firstdoping region or the second doping region.

In certain embodiments, the method further comprises fracturing thecrystalline material substantially along the at least one subsurfacelaser damage pattern to yield first and second crystalline materialportions each having reduced thickness relative to the substrate, butsubstantially a same length and width as the substrate.

In certain embodiments, the substrate comprises silicon carbide. Incertain embodiments, the substrate comprises an ingot having a diameterof at least 150 mm.

In another aspect, the disclosure relates to a material processingapparatus that comprises a laser processing station configured toprocess a substrate of crystalline material, the laser processingstation comprising: a laser configured to form subsurface laser damageregions within an interior of the substrate; at least one translationstage configured to effect relative movement between the laser and thesubstrate; a diffuse light source configured to illuminate a top surfaceof the substrate, wherein the diffuse light source is arranged to bepositioned to a first lateral side of the substrate; and an imagingdevice configured to generate at least one image of the top surface ofthe substrate, wherein the imaging device is configured to be positionedto a second lateral side of the substrate that opposes the first lateralside.

In certain embodiments, the substrate comprises a generally round edgehaving a primary flat, and the diffuse light source is arranged to bepositioned to the first lateral side of the substrate and substantiallyperpendicular to the primary flat.

In certain embodiments, the crystalline material comprises a hexagonalcrystal structure, the diffuse light source is arranged to be positionedto the first lateral side of the substrate and within ±5 degrees ofperpendicular to a <1120> direction of the hexagonal crystal structure.

In certain embodiments, the material processing apparatus furthercomprises a computing device configured to analyze the at least oneimage to identify a condition indicative of presence of uncrackedregions in the interior of the substrate.

In certain embodiments, the computing device is further configured toperform, responsive to the analyzing by the computing device, at leastone of the following steps (i) or (ii): (i) effect relative movementbetween the laser and the substrate while supplying emissions of thelaser focused within the interior of the substrate in at least theuncracked regions to form supplemental subsurface laser damage in thesubstrate and promote formation of additional cracks in the uncrackedregions along or proximate to the first average depth position, forformation of a first reduced thickness portion of the substrate; or (ii)change an instruction set, associated with the substrate, for formingsubsurface laser damage when producing subsurface laser damage patternsat a second average depth position and any subsequent average depthpositions in the substrate, for formation of a second and any subsequentreduced thickness portions of the substrate.

In certain embodiments, the analyzing performed by the computing devicecomprises quantifying a top area property of the one or more uncrackedregions in the interior of the substrate, and comparing the top areaproperty to at least one predetermined threshold area property.

In certain embodiments, the at least one predetermined threshold areaproperty comprises a first predetermined threshold area property and asecond predetermined threshold area property, the second predeterminedthreshold area property being greater than the first predeterminedthreshold area property; the computing device is configured to controlthe material processing apparatus to perform step (ii) if the top areaproperty is at least as large as the first predetermined threshold areaproperty; and the computing device is configured to control the materialprocessing apparatus to perform step (i) if the top area property is atleast as large as the second predetermined threshold area property.

In certain embodiments, the material processing apparatus furthercomprises a memory configured to store the instruction set, associatedwith the substrate, for forming subsurface laser damage in thesubstrate, wherein the memory is accessible to the computing device.

In certain embodiments, the material processing apparatus furthercomprises a fracturing station configured to receive the substrate fromthe laser processing station.

In another aspect, any of the foregoing aspects, and/or various separateaspects and features as described herein, may be combined for additionaladvantage. Any of the various features and elements as disclosed hereinmay be combined with one or more other disclosed features and elementsunless indicated to the contrary herein.

Other aspects, features and embodiments of the present disclosure willbe more fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings incorporated in and forming a part of thisspecification illustrate several aspects of the disclosure, and togetherwith the description serve to explain the principles of the disclosure.

FIG. 1 includes a first frame providing a perspective view of an ingotreceived by a conventional wire saw tool and being subjected to a wiresawing process, and a second frame providing a perspective view ofmultiple wafers obtained by the wire sawing process.

FIG. 2 is a first perspective view crystal plane diagram showing thecoordinate system for a hexagonal crystal such as 4H-SiC.

FIG. 3 is a second perspective view crystal plane diagram for ahexagonal crystal, illustrating a vicinal plane that is non-parallel tothe c-plane.

FIG. 4A is a perspective view wafer orientation diagram showingorientation of a vicinal wafer relative to the c-plane.

FIG. 4B is a simplified cross-sectional view of the vicinal wafer ofFIG. 4A superimposed over a portion of an ingot.

FIG. 5 is a top plan view of an exemplary SiC wafer, with superimposedarrows showing crystallographic orientation directions.

FIG. 6A is a side elevation schematic view of an on-axis ingot ofcrystalline material.

FIG. 6B is a side elevation schematic view of the ingot of FIG. 6A beingrotated by 4 degrees, with a superimposed pattern for cutting endportions of the ingot.

FIG. 6C is a side elevation schematic view of an ingot following removalof end portions to provide end faces that are non-perpendicular to thec-direction

FIG. 7 is a perspective view schematic of a moveable laser toolconfigured to focus laser emissions within an interior of a crystallinematerial to form subsurface damage.

FIGS. 8A and 8B provide exemplary laser tool travel paths relative to acrystalline material for formation of subsurface damage within thecrystalline material, with FIG. 8B including a superimposed arrowshowing orientation of subsurface damage lines relative to the [1120]direction of a hexagonal crystal structure of the crystalline material.

FIG. 9 is a perspective view schematic of the surface structure of anoff-axis (relative to the c-axis) or vicinal 4H-SiC crystal afterfracture but prior to smoothing, with the fractured surface exhibitingterraces and steps.

FIGS. 10A-10D are cross-sectional schematic views of formation ofsubsurface laser damage in a substrate of crystalline material byfocusing laser emissions into a bare substrate, through a surface of asubstrate supported by a carrier, through a carrier and an adhesivelayer into a substrate, and through a carrier into a substrate,respectively.

FIG. 11A provides a top plan view of a crystalline material substrateincluding interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment, with each damagepattern including a plurality of substantially parallel linesperpendicular to the [1120] direction (and substantially perpendicularto a primary substrate flat), and with the laser damage patterns incombination forming multiple three-line groups that are separated fromone another by an inter-group spacing that exceeds a spacing betweenadjacent lines in each three-line group.

FIG. 11B is a top plan schematic view of the crystalline materialsubstrate of FIG. 11A during fabrication, following formation of thefirst subsurface laser damage pattern, with illustration of a firstplurality of cracks within the interior of the substrate propagatinglaterally outward from the first plurality of substantially parallellines.

FIG. 11C is a top plan view of the crystalline material substrate ofFIG. 11B, upon formation of the second subsurface laser damage patternafter the first subsurface laser damage pattern, with illustration of asecond plurality of cracks within the interior of the substratepropagating laterally outward from the second plurality of substantiallyparallel lines but not contacting the first plurality of cracks.

FIG. 11D is a top plan view of the crystalline material substrate ofFIG. 11C, upon formation of the third subsurface laser damage patternafter the first and second subsurface laser damage patterns, withillustration of a third plurality of cracks within the interior of thesubstrate propagating laterally outward from the third plurality ofsubstantially parallel lines and connecting cracks of the firstplurality and second plurality of cracks.

FIG. 12 is a top plan schematic view of a crystalline material substrateincluding interspersed first through third subsurface laser damagepatterns defined therein according to one embodiment similar to thatshown in FIG. 11A, with each damage pattern including a plurality ofsubstantially parallel lines that deviate three degrees relative toperpendicular to the [1120] direction along the substrate surface (andsubstantially perpendicular to a primary substrate flat), and with thelaser damage patterns in combination forming multiple three-line groupsthat are separated from one another by an inter-group spacing thatexceeds a spacing between adjacent lines in each three-line group.

FIG. 13 is a top plan schematic view of a crystalline material substrateincluding interspersed first through fourth laser damage patterns withall lines parallel to one another and perpendicular to the [1120]direction along the substrate surface (and substantially perpendicularto a primary substrate flat).

FIG. 14 is a top plan schematic view of a crystalline material substrateincluding interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment in which first andsecond groups of lines are each parallel to one another andperpendicular to the [1120] direction along the substrate surface (andsubstantially perpendicular to a primary substrate flat), and the thirdgroup of lines is non-parallel to the first and second groups of linesbut does not cross lines of the first and second groups of lines withinthe substrate.

FIG. 15 is a top plan schematic view of a crystalline material substrateincluding interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment in which first andsecond groups of lines are each parallel to one another and deviateabout 3 degrees from perpendicular to the [1120] direction along thesubstrate surface (and substantially perpendicular to a primarysubstrate flat), and the third group of lines is perpendicular to theprimary substrate flat but does not cross lines of the first and secondgroups of lines within the substrate.

FIG. 16 is a top plan schematic view of a crystalline material substrateincluding interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment in which all laserdamage lines are parallel to one another, and inter-group spacing oflaser damage lines is not uniform over at least portions of thesubstrate.

FIG. 17 is a top plan schematic view of a crystalline material substrateincluding interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment in which all laserdamage lines are parallel to one another, and laser damage lines exhibitvariation in intra-group spacing, inter-group spacing, and groupcomposition.

FIG. 18 is a top plan schematic view of a crystalline material substrateincluding sequentially formed first, second, and third subsurface laserdamage patterns defined therein according to one embodiment in whichfirst and second groups of laser damage lines are parallel to oneanother, while a third group of laser damage lines are non-parallel toand cross the first and second groups of laser damage lines.

FIG. 19 is a top plan schematic view of a crystalline material substrateincluding sequentially formed first, second, and third subsurface laserdamage patterns in which each group of laser damage lines includesparallel lines, and each group of laser damage lines is non-parallel toeach other group of laser damage lines.

FIG. 20A is a top plan view of a crystalline material substrateillustrating non-overlapping first, second, and third areas in whichlaser damage regions may be formed.

FIG. 20B is a top plan view of the crystalline material substrate ofFIG. 20A following formation of a first plurality of subsurface laserdamage regions in the first through third areas.

FIG. 20C is a top plan view of the crystalline material substrate ofFIG. 20B following formation of a second plurality of subsurface laserdamage regions in the first through third areas.

FIG. 20D is a top plan view of the crystalline material substrate ofFIG. 20C following formation of a third plurality of subsurface laserdamage regions in the first through third areas.

FIG. 21 is a top plan schematic view of a holder of a laser processingapparatus arranged to hold four substrates in which subsurface laserdamage may be formed with one or more lasers.

FIG. 22A is a top plan schematic view of a single substrate beingprocessed with a split laser beam to simultaneously form subsurfacelaser damage according to a first subsurface laser damage pattern in twoportions of the substrate.

FIG. 22B is a top plan schematic view of a two substrates beingprocessed with a split laser beam to simultaneously form subsurfacelaser damage according to a first subsurface laser damage pattern inboth substrates.

FIG. 23A is a cross-sectional schematic view of a crystalline materialsubstrate including a first subsurface laser damage pattern centered ata first depth.

FIG. 23B is a cross-sectional schematic view of the substrate of FIG.23A following formation of a second subsurface laser damage patterncentered at a second depth and registered with the first subsurfacelaser damage pattern, with an overlapping vertical extent of the firstand second damage patterns.

FIG. 24A is a perspective view photograph of a SiC wafer followingseparation from a thermoplastic glue-bonded sapphire carrier accordingto a method described herein.

FIG. 24B is a perspective view photograph of the sapphire carrier fromwhich the SiC wafer of FIG. 24A was separated.

FIG. 24C is a partially tone-reversed version of the SiC waferphotograph of FIG. 24A to emphasize contrast between a central dopingring and an annular outer portion of the wafer.

FIG. 24D shows the image of FIG. 24C annotated with a dashed-line ovalto denote a boundary between the central doping ring and the annularouter portion of the wafer.

FIG. 25 is a side cross-sectional schematic view of a SiC ingot grown ona seed crystal, showing a cylindrically shaped doping region extendingupward from the seed crystal through the entire thickness of the ingotalong a central portion thereof.

FIG. 26 is a top schematic view of a SiC wafer derived from the SiCingot of FIG. 25 along an illustrated thin sectional portion thereof.

FIG. 27 is a side cross-sectional schematic view of a SiC ingot grown ona seed crystal, showing a frustoconically shaped doping region extendingupward from the seed crystal through the entire thickness of the ingotalong a central portion thereof.

FIG. 28 is a side cross-sectional schematic view of a SiC ingot grown ona vicinal (e.g., offcut) seed crystal, showing a frustoconically shapeddoping region extending upward from the seed crystal at a point offsetfrom a center of seed crystal and upward through the entire thickness ofthe ingot.

FIG. 29 is a perspective view photograph of a Si face of a SiC waferseparated from an ingot by a process involving formation of subsurfacelaser damage and subsequent separation, with an inset portion (upperright) depicting a fragment of the SiC wafer including an edge depictedin subsequent scanning electron microscope (SEM) images.

FIG. 30A is a 45 times magnification SEM image, taken at a 15 degreetilt angle, of a portion of the SiC wafer fragment of FIG. 29, withsuperimposed arrows showing directions of the [1100] and [1120]crystallographic planes.

FIG. 30B is a 1,300 times magnification SEM image, taken at a 15 degreetilt angle, of a portion of the SiC wafer fragment of FIG. 29.

FIG. 30C is a 350 times magnification SEM image, taken at a 15 degreetilt angle, of a portion of the SiC wafer fragment of FIG. 29.

FIG. 30D is a 100 times magnification SEM image taken at a 2 degree tiltangle, of a portion of the SiC wafer fragment of FIG. 29.

FIG. 30E is a 1,000 times magnification SEM image taken at a 2 degreetilt angle, of a portion of the SiC wafer fragment of FIG. 29.

FIG. 31A is a confocal laser scanning microscopy image of a small,central portion of the SiC wafer of FIG. 29, with superimposedcrosshairs marking positions of “trenches” formed by laser scanning.

FIG. 31B is a surface profile plot of the portion of the SiC wafer ofFIG. 31A.

FIG. 32A is a confocal laser scanning microscopy image of a larger,top-proximate (as pictured) portion of the SiC wafer of FIG. 29, withsuperimposed crosshairs marking positions of “trenches” formed by laserscanning.

FIG. 32B is a surface profile plot of the top-proximate portion of theSiC wafer of FIG. 32A.

FIG. 33A is a confocal laser scanning microscopy image of a larger,bottom-proximate (as pictured) portion of the SiC wafer of FIG. 29, withsuperimposed crosshairs marking positions of “trenches” formed by laserscanning.

FIG. 33B is a surface profile plot of the bottom-proximate portion ofthe SiC wafer of FIG. 33A.

FIG. 34A is a side cross-sectional schematic view of a solid carrierhaving adhesive material joined to a surface thereof.

FIG. 34B is a cross-sectional schematic view of an assembly includingthe solid carrier and adhesive material of FIG. 34A joined to acrystalline material substrate having a subsurface laser damage regionproximate to the adhesive material lip.

FIG. 34C is a cross-sectional schematic view of the assembly of FIG.34B, with a surface of the solid carrier being positioned on a coolingapparatus in the form of a liquid-cooled chuck.

FIG. 34D is a cross-sectional schematic view of a majority of thecrystalline material substrate separated from a bonded assembly (atopthe liquid-cooled chuck) including the solid carrier and a portion ofthe crystalline material removed from the substrate, following fractureof the crystalline material along the subsurface laser damage region.

FIG. 34E is a cross-sectional schematic view of the bonded assembly ofFIG. 34D following removal from the liquid-cooled chuck, with residuallaser damage along an upward facing surface.

FIG. 34F is a cross-sectional schematic view of the portion of thecrystalline material supported by a heated vacuum chuck, with the solidcarrier and adhesive material being laterally translated away from thecrystalline material portion following thermal softening and release ofthe adhesive material.

FIG. 35 is a cross-sectional schematic view of a crystalline materialhaving subsurface laser damage and bonded to a rigid carrier, with thecrystalline material and carrier arranged in a liquid bath of anultrasonic generator.

FIGS. 36A-36C are cross-sectional schematic views illustrating steps forfracturing a crystalline material having subsurface laser damageincluding application of a mechanical force proximate to one edge of acarrier to impart a bending moment in at least a portion of the carrier.

FIGS. 37A-37O are cross-sectional schematic views illustrating steps ofa device wafer splitting process, according to which a thick wafer isfractured from a crystalline material, at least one epitaxial layer isgrown on the thick wafer, and the thick wafer is fractured to form afirst and second bonded assemblies each including a carrier and a thinwafer divided from the thick wafer, with the first bonded assemblyincluding the at least one epitaxial layer as part of an operativesemiconductor-based device.

FIG. 38 is a flowchart schematically illustrating steps for producingsubsurface laser damage and bonding a rigid carrier to a crystalline(e.g., SiC) material ingot, followed by laser parting of a bondedassembly including the carrier and a portion of the crystallinematerial, followed by further processing of the bonded assembly andformation of epitaxial layers on a device wafer, with return of theingot and the rigid carrier to a beginning of the process.

FIG. 39 is a cross-sectional schematic view of a portion of thecrystalline material substrate of FIG. 38 showing subsurface laserdamage with superimposed dashed lines identifying an anticipated kerfloss material region attributable to laser damage and subsequent surfaceprocessing (e.g., grinding and planarization).

FIG. 40 is a schematic illustration of a material processing apparatusaccording to one embodiment, including a laser processing station, amaterial fracturing station, multiple coarse grinding stations arrangedin parallel, a fine grinding station, and a CMP station.

FIG. 41 is a schematic illustration of a material processing apparatusaccording to one embodiment similar to that of FIG. 40, but with an edgegrinding station arranged between the fine grinding station and thecoarse grinding stations.

FIG. 42 is a schematic illustration of a material processing apparatusaccording to one embodiment, including a laser processing station, amaterial fracturing station, multiple coarse grinding stations arrangedin parallel, a fine grinding station, a surface coating station, an edgegrinding station, a coating removal station, and a CMP station.

FIG. 43A is a schematic side cross-sectional view of a first apparatusfor holding an ingot having end faces that are non-perpendicular to asidewall thereof, according to one embodiment.

FIG. 43B is a schematic side cross-sectional view of a second apparatusfor holding an ingot having end faces that are non-perpendicular to asidewall thereof, according to one embodiment.

FIG. 44 is a schematic side cross-sectional view of a conventional laserfocusing apparatus that focuses an incoming horizontal beam with a lens,forming an outgoing beam having a beam waist pattern having a minimumwidth at a downstream position corresponding to a focal length of thelens.

FIG. 45 is a schematic side cross-sectional view of a verticallyoriented focused laser beam exhibiting a beam waist within a crystallinematerial, with illustration of decomposition threshold points atdifferent vertical positions relative to the beam waist.

FIGS. 46A-46C provide plots of laser power versus sequential waferidentification for wafers derived from three SiC ingots, respectively,showing an increase in laser power with wafer identification number.

FIG. 47 is a plot of resistivity (Ohm-cm) versus slice number for fiftywafers produced from a SiC ingot with a superimposed polynomial fitshowing resistivity decreasing with slice number, wherein an increasingslice number represents increasing proximity to a seed crystal on whichthe ingot was grown (e.g., via a physical vapor transport (PVT)process).

FIG. 48 is a plot of laser power (Watts) versus resistivity for thewafers produced from the SiC ingot of FIG. 47, with a superimposedpolynomial fit showing a decrease in laser power required to achievewith increasing resistivity values.

FIGS. 49A and 49B provide schematic side cross-sectional and top planviews, respectively, of a diffuse light source and an imaging devicearranged proximate to a substrate within a laser processing station.

FIG. 50A is an image of a top surface of a crystalline SiC substratehaving subsurface laser damage, showing regions of different colors andirregularly shaped dark regions corresponding to uncracked regionswithin the substrate.

FIG. 50B is a schematic diagram of a substrate representation showingthe irregularly shaped dark regions of FIG. 50A within dotted lineregions substantially corresponding to boundaries betweendifferently-colored regions of the top surface of the substrate of FIG.50A.

FIG. 50C is a magnified view of the irregularly shaped dark regions ofFIGS. 50A-50B with addition of rectangular boxes around individualregions.

FIG. 51 is a schematic illustration of a material processing apparatusaccording to one embodiment, including a laser processing station thatincludes a laser, at least one translation stage, a diffuse light sourceconfigured to illuminate a top surface of a substrate, and an imagingdevice configured to generate at least one image of the top surface ofthe substrate.

FIG. 52 is a flowchart illustrating steps in a first crystallinematerial processing method that includes generating an image of a topsurface of a substrate having subsurface laser damage, analyzing theimage to identify presence of a condition indicative of one or moreuncracked regions, comparing one or more properties of the uncrackedregions to first and second thresholds, and taking action (i.e., (A)performing an additional laser pass at substantially the same depthposition to form supplemental laser damage, optionally with adjustmentof one or more laser parameters and/or (B) adjusting one or more laserparameters for forming subsurface laser damage at second and subsequentdepth positions) responsive to the comparisons to enhance reliability ofproducing substrate portions (e.g., wafers) from the substrate.

FIG. 53 is a flowchart illustrating steps in a second crystallinematerial processing method that includes generating an image of a topsurface of a substrate having subsurface laser damage, analyzing theimage to quantify a top area property of one or more uncracked regions,comparing the top area property to first and second threshold areaproperties, and taking action (i.e., performing an additional laser passat the same depth position and/or adjusting power for subsurface laserdamage at subsequent depth positions) responsive to the comparisons toenhance reliability of producing substrate portions (e.g., wafers) fromthe substrate.

FIG. 54 is a schematic diagram of a generalized representation of acomputer system that can be included in any component of the systems ormethods disclosed herein.

DETAILED DESCRIPTION

The present disclosure relates in various aspects to methods forprocessing a crystalline material substrate and a material processingapparatus. A crystalline material processing method includes generatingsubsurface laser damage sites in areas of the crystalline material at afirst average depth position to promote formation of cracks in thesubstrate interior propagating outward from a subsurface laser damagepattern, imaging the substrate top surface, analyzing the image toidentify a condition indicative of presence of uncracked regions withinthe substrate, and taking one or more actions responsive to theanalyzing (e.g., upon attainment of appropriate conditions). Onepotential action includes forming supplemental subsurface laser damageat the first average depth position to promote formation of additionalcracks in the uncracked regions, for formation of a first reducedthickness portion of the substrate (e.g., a first wafer). Anotherpotential action includes changing an instruction set for producingsubsequent laser damage formation (at second or subsequent average depthpositions, for formation of second and any reduced thickness portions ofthe substrate), without necessarily forming additional damage at thefirst average depth position. The laser damage facilitates subsequentfracture of the substrate to yield multiple substrate portions ofreduced thickness.

In certain embodiments, the analyzing comprises quantifying a top areaproperty of the one or more uncracked regions in the interior of thesubstrate, and comparing the top area property to at least onepredetermined threshold area property. In certain embodiments, if afirst threshold area property is exceeded, then average laser power isincrementally increased in a subsequent laser damage formation step(i.e., at second or subsequent average depth positions, for formation ofsecond and subsequent reduced thickness substrate portions), withoutnecessarily forming additional damage at the first average depthposition. As an alternative to, or in addition to, increasing laserpower, a laser focusing depth relative to a top surface may be alteredand/or a number of laser damage formation passes may be altered, in aninstruction set for performing a second and subsequent laser damageformation step. If a second, greater threshold area property is exceeded(suggesting that uncracked regions may be large enough to impedefracturing), then supplemental subsurface laser damage is formed at thefirst average depth position, to supplement the at least one subsurfacelaser damage pattern and promote formation of additional cracks in theuncracked regions along or proximate to the first average depthposition, for formation of a reduced thickness portion of the substrate.This supplemental damage may be formed before the substrate is removedfrom the laser processing station, thereby enhancing laser processingstation throughput by avoiding superfluous substrate demounting andre-mounting steps.

In additional aspects, the present disclosure relates to a materialprocessing apparatus that comprises a laser processing stationconfigured to process a substrate of crystalline material, the laserprocessing station comprising: a laser configured to form subsurfacelaser damage regions within an interior of the substrate; at least onetranslation stage configured to effect relative movement between thelaser and the substrate; a diffuse light source configured to illuminatea top surface of the substrate, wherein the diffuse light source isarranged to be positioned to a first lateral side of the substrate; andan imaging device configured to generate at least one image of the topsurface of the substrate, wherein the imaging device is configured to bepositioned to a second lateral side of the substrate that opposes thefirst lateral side. Such apparatus causes uncracked regions adjacent tosubsurface laser damage within a substrate interior to be visible at asurface thereof as dark (e.g., black or nearly black) spots on a topsurface of the substrate. Such apparatus also causes regions havingdifferent degrees of cracking among subsurface laser damage areas toexhibit different colors at the top surface of the substrate. Since darkspots typically appear first in the facet area (corresponding to thedoping ring), in certain embodiments, the facet area can be isolated.

As mentioned previously, variations in material and/or opticalproperties within a thick substrate (such as a SiC ingot), and alsoamong different ingots of the same composition, render it challenging toeasily and reproducibly fabricate wafers of uniform thickness by laserprocessing while avoiding unnecessary material loss. Applicant has foundthat, when wafers are sequentially formed from SiC ingots by formationof subsurface laser damage followed by fracturing, it is necessary toincrease laser power as damage formation progresses in depth position toenable successful fracture. (Restated, when forming multiple wafers froma SiC ingot, initial wafers distal from the seed crystal may besuccessfully parted following formation of laser damage produced at alower average laser power, but progressively higher laser power levelsbecome necessary for laser damage used to part subsequent wafers as thegrowth position of the parted wafers gets closer to the seed crystal.)This behavior is believed to be primarily driven by bulk opticalabsorption changes, but may also be influenced by other changes in thecrystal lattice. One theoretical solution to this problem would be tosimply use high laser power at each sequential depth position whenforming subsurface damage, but this would result in unnecessary materialloss when damage is produced “early” in the ingot (e.g., at the firstseveral depth positions distal from the seed crystal), and would alsosignificantly increase wafer-to-wafer thickness spread due tovariability in both the damage depth and the point at whichdecomposition is reached relative to a laser beam waist (resulting fromthe focal length of a beam focusing optic). Trying to constantly adjustfor wafer thickness is neither practical nor accurate due to measurementinaccuracies caused by rough surfaces produced by the laser separationprocess, and due to the relationship between laser depth and requiredlaser power.

Before detailing specific features of the foregoing method and apparatus(with particular embodiments being described in connection with FIGS. 45to 51), apparatuses and methods for processing crystalline materialsubstrates will be introduced.

Terminology and Definitions

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

As used herein, a “substrate” refers to a crystalline material, such asa single crystal semiconductor material, optionally comprising an ingotor a wafer, that is divisible into at least two thinner portions havingsubstantially the same lateral dimensions (e.g., diameter, or length andwidth) as the substrate, and having sufficient thickness (i) to besurface processed (e.g., lapped and polished) to support epitaxialdeposition of one or more semiconductor material layers, and optionally(ii) to be freestanding if and when separated from a rigid carrier. Incertain embodiments, a substrate may have a generally cylindrical shape,and/or may have a thickness of at least about one or more of thefollowing thicknesses: 300 μm, 350 μm, 500 μm, 750 μm, 1 mm, 2 mm, 3 mm,5 mm, 1 cm, 2 cm, 5 cm, 10 cm, 20 cm, 30 cm, or more. In certainembodiments, a substrate may include a thicker wafer that is divisibleinto two thinner wafers. In certain embodiments, a substrate may be partof a thicker wafer having one or more epitaxial layers (optionally inconjunction with one or more metal contacts) arranged thereon as part ofa device wafer with a plurality of electrically operative devices. Thedevice wafer may be divided in accordance with aspects of the presentdisclosure to yield a thinner device wafer and a second thinner wafer onwhich one or more epitaxial layers (optionally in conjunction with oneor more metal contacts) may be subsequently formed. In certainembodiments, a substrate may comprise a diameter of 150 mm or greater,or 200 mm or greater. In certain embodiments, a substrate may comprise4H—SiC with a diameter of 150 mm, 200 mm, or greater, and a thickness ina range of 100 to 1000 microns, or in a range of 100 to 800 microns, orin a range of 100 to 600 microns, or in a range of 150 to 500 microns,or in a range of 150 to 400 microns, or in a range of 200 to 500microns, or in any other thickness range or having any other thicknessvalue specified herein.

The terms “first average depth position,” “second average depthposition,” and “subsequent average depth position” as used herein referto depth positions (e.g., horizontal planes) within a substrate, asmeasured from an initial top surface of the substrate, for formation ofreduced thickness portions of the substrate. For example, a firstaverage depth position may correspond to a subsurface laser damageposition for forming a first wafer from an ingot, a second average depthposition may correspond to a subsurface laser damage position forforming a second wafer from the ingot, and so on. In certainembodiments, each reduced thickness portion derived from the substratehas the same or substantially the same thickness. The term “averagedepth position” is used instead of depth position in recognition of thefact that in certain embodiments, a laser focusing depth may be subjectto small differences between passes or even within a single pass forformation of a laser damage pattern for formation of a single reducedthickness portion of a substrate (e.g., one wafer), with such smalldifferences preferably being in a range of 1 to 10 microns, or 2 to 8microns, or 2 to 6 microns. This is to be distinguished from the muchgreater difference between first and second average depth positions,which is typically in a range of at least 100 microns (or at least 150microns, 200 microns, 300 microns, 400 microns, 500 microns or more).

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawings, thoseskilled in the art will understand the concepts of the disclosure andwill recognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

Materials

Methods disclosed herein may be applied to substrates of variouscrystalline materials, of both single crystal and polycrystallinevarieties. In certain embodiments, methods disclosed herein may utilizecubic, hexagonal, and other crystal structures, and may be directed tocrystalline materials having on-axis and off-axis crystallographicorientations. In certain embodiments, methods disclosed herein may beapplied to semiconductor materials and/or wide bandgap materials.Exemplary materials include, but are not limited to, Si, GaAs, anddiamond. In certain embodiments, such methods may utilize single crystalsemiconductor materials having hexagonal crystal structure, such as4H-SiC, 6H-SIC, or Group III nitride materials (e.g., GaN, AlN, InN,InGaN, AlGaN, or AlInGaN). Various illustrative embodiments describedhereinafter mention SiC generally or 4H-SiC specifically, but it is tobe appreciated that any suitable crystalline material may be used. Amongthe various SiC polytypes, the 4H-SiC polytype is particularlyattractive for power electronic devices due to its high thermalconductivity, wide bandgap, and isotropic electron mobility. Bulk SiCmay be grown on-axis (i.e., with no intentional angular deviation fromthe c-plane thereof, suitable for forming undoped or semi-insulatingmaterial) or off-axis (typically departing from a grown axis such as thec-axis by a non-zero angle, typically in a range of from 0.5 to 10degrees (or a subrange thereof such as 2 to 6 degrees or anothersubrange), as may be suitable for forming N-doped or highly conductivematerial). Embodiments disclosed herein may apply to on-axis andoff-axis crystalline materials, as well as doped and unintentionallydoped crystalline semiconductor materials. Doped semiconductor material(e.g., N-doped SiC) exhibits some infrared absorption, thus requiringthe use of higher laser power than undoped material to impart subsurfacelaser damage. In certain embodiments, crystalline material may includesingle crystal material, and may further include single crystalsemiconductor material. Certain embodiments disclosed herein may utilizeon-axis 4H—SiC or vicinal (off-axis) 4H—SiC having an offcut in a rangeof from 1 to 10 degrees, or from 2 to 6 degrees, or about 4 degrees.

Certain embodiments herein may use substrates of doped or undoped SiC,such as SiC ingots (also known as boules), which may be grown byphysical vapor transport (PVT) or other conventional ingot fabricationmethods. If doped SiC is used, such doping may render the SiC N-type orsemi-insulating in character. In certain embodiments, a N-type SiC ingotis intentionally doped with nitrogen. In certain embodiments, a N-typeSiC ingot includes resistivity values within a range of 0.015 to 0.028Ohm-cm. In certain embodiments, a SiC ingot may have resistivity valuesthat vary with vertical position, such that different substrate portions(e.g., wafers) have different resistivity values, which may be due tovariation in bulk doping levels during ingot growth. In certainembodiments, a SiC ingot may have doping levels that vary horizontally,from a higher doping region proximate to a center of the ingot to alower doping level proximate to a lateral edge thereof. Variation iningot doping and resistivity with respect to vertical and horizontalposition may render it necessary to adjust laser damage formationparameters for formation of different reduced thickness portions (e.g.,wafers) of a substrate (e.g., an ingot) and/or during formation of asingle reduced thickness portion of a substrate. In certain embodiments,resistivity is greatest proximate to an exposed surface of an ingot, andis lowest proximate to a growth seed. A reduction in resistivitycorresponds to an increase in doping and an increase in laserabsorption.

FIGS. 6A and 6C schematically illustrate on-axis and off-axiscrystalline substrates in the form of ingots that may be utilized withmethods disclosed herein. FIG. 6A is a side elevation schematic view ofan on-axis ingot 15 of crystalline material having first and second endfaces 16, 17 that are perpendicular to the c-direction (i.e., [0001]direction for a hexagonal crystal structure material such as 4H—SiC).FIG. 6B is a side elevation schematic view of the ingot 15 of FIG. 6Abeing rotated by four degrees, with a superimposed pattern 18 (shown indashed lines) for cutting and removing end portions of the ingot 15proximate to the end faces 16, 17. FIG. 6C is a side elevation schematicview of an off-axis ingot 15A formed from the ingot 15 of FIG. 6B,following removal of end portions to provide new end faces 16A, 17A thatare non-perpendicular to the c-direction. If laser emissions of a firstdepth are supplied through an end face 16 of the ingot 15 to formsubsurface laser damage, a carrier (not shown) is joined to the end face16, and the ingot 15 is fractured along the subsurface laser damage,then an on-axis wafer may be formed. Conversely, if laser emissions of afirst depth are supplied through an end face 16A of the off-axis ingot15A to form subsurface laser damage, a carrier (not shown) is joined tothe end face 16A, and the ingot 15A is fractured along the subsurfacelaser damage, then an off-axis wafer may be formed.

Subsurface Laser Damage Formation

Processing of a crystalline material substrate to form multiple patternsof subsurface laser damage facilitates subsequent fracture of thesubstrate to yield reduced thickness first and second crystallinematerial portions of the substrate. Certain methods involveinterspersing of multiple sequentially formed pluralities ofsubstantially parallel lines of multiple subsurface laser damagepatterns, respectively, wherein at least some lines of a second (e.g.,subsequently formed) plurality of lines do not cross lines of a firstplurality of lines. Certain methods involve formation of initial andsubsequent subsurface laser damage patterns each comprising a pluralityof substantially parallel lines in a substrate of crystalline material,with lines of the initial and subsequent pluralities of substantiallyparallel lines being non-parallel to one another, wherein an angulardirection of lines of the subsequent plurality of substantially parallellines differs by no more than 10 degrees from an angular direction oflines of the initial plurality of substantially parallel lines, and atleast some lines of the subsequent plurality of substantially parallellines do not cross any lines of the initial plurality of substantiallyparallel lines. Certain methods involve formation of an initialsubsurface laser damage pattern substantially centered at an initialdepth within an interior of a crystalline material of a substrate, andformation of a subsequent subsurface laser damage pattern substantiallycentered at a subsequent depth (differing from the initial depth) withinthe substrate, wherein the subsequent subsurface laser damage pattern issubstantially registered with the initial subsurface laser damagepattern, and vertical extents of at least portions of the initial andsubsurface laser damage patterns are overlapping.

Sequential formation of interspersed or interleaved subsurface laserdamage patterns distributed over a crystalline material is believed tobeneficially maintain sufficient stress within a crystalline material tofacilitate subsequent material fracture using methods herein, whileenabling high laser tool throughput in conjunction with modest materialdamage and concomitantly low kerf losses. It would be simple inprinciple to use high laser power and scan nearly an entirety of acrystalline material to facilitate fracturing along a laser damage line.Such an approach can reliably separate thin layers of crystallinematerial from a bulk substrate (e.g., an ingot), but high laser powertends to increase material damage, necessitating significant surfaceprocessing (e.g., grinding and planarization) to remove the damage.Close spacing between laser damage lines will help promote fracture, butat the cost of significantly reducing throughput of a laser processingtool. A conventional approach for forming subsurface laser damage hasinvolved forming a subsurface laser damage line in a forward directionacross a crystalline material, followed by relative indexing in alateral direction between the material and a laser, followed by forminga subsurface laser damage line in a rearward direction, followed bylateral indexing in the same lateral direction, and so on. Such approachgenerally requires higher laser power or closer spacing betweensequentially formed laser damage lines, which will tend to reducethroughput or impart a greater degree of damage, thereby increasing kerfloss due to the need to remove additional material from laser-processedsurfaces for removal of the laser damage. This conventional approachdoes not involve forming a first distributed subsurface laser damagepattern (e.g., involving formation of a first plurality of laser damageregions over multiple non-overlapping areas of a substrate) followed byformation of a second distributed subsurface laser damage pattern (e.g.,involving formation of a second plurality of laser damage regions overthe same multiple non-overlapping areas of the substrate), with thesecond subsurface laser damage pattern being interleaved or interspersedamong the first subsurface laser damage pattern.

Various embodiments disclosed herein address the concern of promotingreliable separation of thin layers (e.g., wafers) of crystallinematerial from a substrate without unduly high laser power, whileenabling high laser tool throughput and providing low kerf losses.Certain embodiments herein involve forming an initial distributedsubsurface laser damage pattern in a crystalline material substrate(e.g., over each area of a plurality of non-overlapping areas of thesubstrate), then forming at least one subsequent distributed subsurfacelaser damage pattern in the same substrate (e.g., over each area of thesame plurality of non-overlapping areas), wherein at least portions(e.g., lines) of the at least one subsequent laser damage pattern arearranged in gaps between laser damage lines of the initial laser damagepattern, thereby providing interspersed or interleaved subsurface laserdamage patterns. In certain embodiments, at least some (or all) laserdamage lines of at least one subsequently formed laser damage pattern donot cross laser damage lines of an initial subsurface laser damagepattern. It is believed that non-crossing of laser damage patterns maybeneficially avoid localized stresses from being dissipated. In certainembodiments, first and second interspersed subsurface laser damagepatterns are formed in such a manner to prevent propagation of localizedsubsurface cracks therebetween, but application of a third (orsubsequent) interspersed subsurface laser damage pattern will causelocalized subsurface cracks to propagate and join in a substantiallycontinuous manner over an entire internal plane of a crystallinematerial substrate, thereby easing subsequent fracture along the laserdamage region using techniques disclosed herein. Formation ofinterspersed subsurface laser damage according to methods describedherein has been observed to permit reliable separation of thin layers ofcrystalline material from a substrate with a smaller number of laserdamage lines per layer to be removed, beneficially providing increasedlaser tool throughput while providing low levels of laser damage(enabling low kerf losses).

Various embodiments refer to laser subsurface damage including linesthat are oriented relative to a crystal structure of a substrate. Incertain embodiments, a substrate comprises a crystalline material havinga hexagonal crystal structure, wherein laser damage lines are orientedperpendicular to, or within ±5 degrees of perpendicular to, a <1120>direction of the hexagonal crystal structure and parallel orsubstantially parallel to (e.g., within ±5 degrees, ±3 degrees, or ±1degree of) a surface of the substrate. Although a primary flat on aconventional 4H—SiC wafer is intended to be oriented parallel to the<1120> direction of the hexagonal crystal structure, a primary flat maynot be truly parallel to such direction due to variations inmanufacturing. Various SiC wafer manufacturers provide publishedspecifications for primary flat orientation of ±5 degrees from parallelto the <1120> direction of the hexagonal crystal structure. It istherefore preferred to use x-ray diffraction (XRD) data rather thanwafer flat alignment to determine proper laser orientation for formationof subsurface laser damage.

Tools for forming laser subsurface damage in crystalline materials areknown in the art and commercially available from various providers, suchas Disco Corporation (Tokyo, Japan). Such tools permit laser emissionsto be focused within an interior of a crystalline material substrate,and enable lateral movement of a laser relative to the substrate.Typical laser damage patterns in the art include formation of parallellines that are laterally spaced relative to one another at a depthwithin a crystalline substrate. Parameters such as focusing depth, laserpower, translation speed, and subsurface damage line spacing may beadjusted to impart laser damage, but adjustment of certain factorsinvolves tradeoffs. Increasing laser power tends to impart greatersubsurface damage that may enhance ease of fracturing (e.g., by reducingthe stress required to complete fracturing), but greater subsurfacedamage increases surface irregularities along surfaces exposed byfracturing, such that additional processing may be required to rendersuch surfaces sufficiently smooth for subsequent processing (e.g., forincorporation in electronic devices), and the additional processingleads to additional kerf losses. Reducing lateral spacing betweensubsurface laser damage lines may also enhance ease of fracturing, but areduction in spacing between laser damage lines increases the number oftranslational passes between a substrate and a laser, thereby reducingtool throughput.

FIG. 7 is a perspective view schematic of one example of a laser tool 29configured to focus laser emissions within an interior of a crystallinematerial 30 to form subsurface damage 40. The crystalline material 30includes an upper surface 32 and an opposing lower surface 34, and thesubsurface damage 40 is formed in the interior of the crystallinematerial 30 between the upper and lower surfaces 32, 34. Laser emissions36 are focused with a lens assembly 35 to yield a focused beam 38, witha focal point thereof being in the interior of the crystalline material30. Such laser emissions 36 may be pulsed at any suitable frequency(typically in the nanosecond, picosecond, or femtosecond range) and beamintensity, with a wavelength below the bandgap of the crystallinematerial 30 to permit the laser emissions 36 to be focused at a targeteddepth below a surface thereof. At the focal point, the beam size andshort pulse width results in an energy density high enough to result invery localized absorption that forms subsurface damage. One or moreproperties of the lens assembly 35 may be altered to adjust a focalpoint of the focused beam 38 to a desired depth within the crystallinematerial 30. Relative lateral motion (e.g., lateral translation) betweenthe lens assembly 35 and the crystalline material 30 may be effected topropagate the subsurface damage 40 in a desired direction, asschematically illustrated by dashed line 44. Such lateral movement maybe repeated in various patterns, including patterns as describedhereinafter.

FIGS. 8A and 8B provide exemplary laser tool travel paths relative to acrystalline material for formation of subsurface damage within thecrystalline material. In certain embodiments, a laser tool portion(e.g., including a lens assembly) may be configured to move while acrystalline material is stationary; in other embodiments, a laser toolportion may be held stationary while a crystalline material is movedrelative to the tool portion. FIG. 8A shows a reversing y-directionlinear scanning movement 46 suitable for forming subsurface damage in apattern of laterally spaced parallel lines within a first crystallinematerial 45A. FIG. 8B shows y-direction linear scanning movement 48 over(and beyond) an entire surface of a crystalline material 45B (withslight advancement in an x-direction upon each reversal in y-direction),sufficient to form parallel subsurface laser damage lines distributedthrough the crystalline material 45B. As shown, the laser damage linesare perpendicular to the [1120] direction of a hexagonal crystalstructure of the crystalline material 45B along a surface of thecrystalline material 45B, and are and substantially parallel to thesurface of the crystalline material 45B.

Coverage of an entire surface of a crystalline material with laser linesformed in a y-direction, with unidirectional advancement in thex-direction following each y-direction reversal, may be referred to as asingle pass of laser damage formation. In certain embodiments, laserprocessing of crystalline material to form subsurface damage may beperformed in two, three, four, five, six, seven, or eight passes, or anyother suitable number of passes. Increasing the number of passes atlower laser power can reduce kerf losses. To achieve a desirable balanceof material loss versus process speed, desirable numbers of lasersubsurface damage formation passes have been found to be two to fivepasses, or three to four passes, prior to performance of a fracturingstep.

In certain embodiments, lateral spacing between adjacent lasersubsurface damage lines (whether formed in a single pass or multiplepasses) may be in a range of from 80 to 400 microns, or from 100 to 300microns, or from 125 to 250 microns. Lateral spacing between adjacentlaser subsurface damage lines impacts laser processing time, ease offracture, and (depending on c-plane orientation or misorientation)effective laser damage depth.

It has been observed that forming subsurface laser damage lines incrystalline material results in formation of small cracks in theinterior of the material propagating outward (e.g., laterally outward)from the laser damage lines. Such cracks appear to extend substantiallyor predominantly along the C-plane. The length of such cracks appear tobe functionally related to laser power level (which may be calculated asthe product of pulse frequency times energy per pulse). For adjacentlaser subsurface damage lines spaced apart by a specific distance, ithas been observed that increasing laser power in forming such lasersubsurface damage lines tends to increase the ability of cracks toconnect or join between the laser subsurface damage lines, which isdesirable to promote ease of fracturing.

If the crystalline material subject to laser damage formation includesan off-axis (i.e., non c-plane) orientation (e.g., in a range of from0.5-10 degrees, 1-5 degrees, or another misorientation), suchmisorientation may affect desirable laser damage line spacing.

A SiC substrate may include surfaces that are misaligned (e.g., off-axisat an oblique angle relative to) the c-plane. An off-axis substrate mayalso be referred to as a vicinal substrate. After fracturing such asubstrate, the as-fractured surface may include terraces and steps(which may be smoothed thereafter by surface processing such as grindingand polishing). FIG. 9 is a perspective view schematic of a surfacestructure of an off-axis 4H—SiC crystal 50 (having an angle A relativeto a c-axis basal plane) after fracture but prior to smoothing. Thefractured surface exhibits steps 52 and terraces 54 relative to a c-axisbasal plane 56. For a 4 degree off-axis surface, steps theoreticallyhave a height of about 17 microns for a terrace width of 250 microns.For a 4H—SiC crystal having subsurface laser damage, 250 micron spacingbetween laser lines forms terraces of 250 micron width. Afterfracturing, the stepped surface is subject to being ground smooth,planarized, and polished in preparation for epitaxial growth of one ormore layers thereon.

When subsurface laser damage is formed in crystalline material (e.g.,SiC), and if subsurface laser damage lines are oriented away fromperpendicular to a substrate flat (i.e., non-perpendicular to the [1120]direction), then such laser damage lines extend through multiple stepsand terraces in a manner equivalent to off-axis semiconductor material.For purposes of subsequent discussion, the term “off-axis lasersubsurface damage lines” will be used to refer to laser subsurfacedamage lines that are non-perpendicular to the [1120] direction.

Providing spacing that is too large between adjacent subsurface laserdamage lines inhibits fracture of crystalline material. Providingspacing that is too small between adjacent subsurface laser damage linestends to reduce step heights, but increases the number of verticalsteps, and increasing the number of vertical steps typically requiresgreater separation force to complete fracturing.

Reducing spacing between adjacent laser damage lines to a distance thatis too small may yield diminishing returns and substantially increaseprocessing time and cost. A minimum laser energy threshold is requiredfor SiC decomposition. If this minimum energy level creates connectedcracks between two laser lines spaced about 100 microns apart, thenreducing laser line spacing below this threshold likely offers littlebenefit in terms of reducing kerf loss.

Surface roughness of crystalline material exposed by fracturing canimpact not only subsequent handling such as robot vacuum, but also grindwheel wear, which is a primary consumable expense. Roughness is impactedby both the spacing of subsurface laser damage lines and orientation ofsuch subsurface damage lines relative to the crystal structure of thesemiconductor material. Reducing a gap between subsurface damage linessimply reduces potential step height. Providing off-axis lasersubsurface damage lines tends to breaks up the long parallel steps thatwould otherwise be present at the laser damage region, and it also helpsmitigate at least some impact from C plane slope or curvature. When thelaser lines are perpendicular to the flat of a substrate, the cleaveplane parallel to the laser lines along the C plane extends about 150 mmfrom the flat to the opposing curved end of the wafer. Slight deviationsin the C plane slope or curvature (which are common for SiC substrates)can create significant variability in the fractured surface as it forcesplane jumping as a fracture propagates. A drawback to providing off-axislaser subsurface damage lines is that such subsurface damage linesgenerally require laser power to be increased to form connected cracksbetween adjacent laser lines. Thus, in certain embodiments, forming acombination of on-axis subsurface laser damage lines (that areperpendicular to the primary flat) and off-axis laser subsurface damagelines provides a good balance between avoiding excessive variability inthe fractured surface without requiring unduly increased laser power toform connected cracks between adjacent laser lines.

In certain embodiments, a laser having a wavelength of 1064 nm may beused to implement methods disclosed herein, with the inventors havinggained experience in processing of 4H—SiC. Although a wide range ofpulse frequencies may be used in certain embodiments, pulse frequenciesof 120 kHz to 150 kHz have been successfully employed. A translationstage speed of 936 mm/s between a laser and a substrate to be processedhas been successfully utilized; however, higher or lower translationstage speeds may be used in certain embodiments with suitable adjustmentof laser frequency to maintain desirable laser pulse overlap. Averagelaser power ranges for forming subsurface laser damage in doped SiCmaterial are in a range of from 3 W to 8 W, and 1 W to 4 W for undopedSiC material. Laser pulse energy may be calculated as power divided byfrequency. Laser pulse widths of 3 ns to 4 ns may be used, althoughother pulse widths may be used in other embodiments. In certainembodiments, a laser lens Numerical Aperture (NA) in a range of 0.3 to0.8 may be used. For embodiments directed to processing of SiC, giventhe refractive index change going from air (˜1) to SiC (˜2.6), asignificant change in refractive angle is experience inside SiC materialto be processed, making laser lens NA and aberration correctionimportant to achieving desirable results.

One of the primary drivers of kerf loss is subsurface laser damage belowthe primary fracture region on the ingot side. In general, an increasein subsurface laser damage increases kerf loss. One potential cause ofincreased subsurface laser damage is a failure to adequately compensatefor the optical characteristics of the crystalline material. In certainembodiments, optical parameter optimization may be periodicallyperformed (e.g., each time a crystalline material substrate (e.g.,ingot) is supplied to the laser tool) prior to formation of subsurfacelaser damage in a substrate. Such optimization may utilize variableheight adjustment to attainment of an initial state in which a bestfocus point of the laser beam is formed an upper surface of thecrystalline material substrate, followed by adjusting the apertureand/or correction collar adjustment ring of the laser tool correspondingto a desired depth of formation of subsurface laser damage in thecrystalline material according to a subsequent state.

In certain embodiments, a crystalline material substrate may exhibitdoping that varies with respect to position (e.g., laterally and/or withdiameter) across a primary surface (e.g., face) of the substrate. Dopantdensity is usually higher in a central region of a SiC {0001} wafer, asobservable by a darker color in such region. This increased dopantdensity is due to enhanced impurity incorporation that occurs duringfacet growth. During growth of a SiC {0001} ingot, a {0001} facetappears near the center of the ingot. On the {0001} facet, fast spiralgrowth takes place, but crystal growth rates along the <0001> directionis relatively slow. Therefore, impurity concentration is enhanced alongthe {0001} facet region. The dopant density at the center (i.e., thefacet region) of a SiC wafer may be 20% to 50% higher than a dopantdensity outside this region. Formation of a doping ring region ofincreased dopant concentration in SiC is shown in FIGS. 14A, 14C, and14D. Such region exhibits higher laser absorption and slightly alteredrefractive index, wherein both of the foregoing phenomena impact depthof focusing of laser emissions in the substrate. Increasing laser powerwhen impinging focused laser emissions into the doping ring region,relative to power used when impinging focused laser emissions into thematerial outside the doping ring region, can compensate for differingproperties of the doping ring region. In certain embodiments, presenceof a condition indicative of non-uniform doping of a crystallinematerial across at least a portion of a surface of the substrate may bedetected to determine presence of at least one first doping region andat least one second doping region. (Methods for detecting differentdoping conditions include, but are not limited to, interferometry,resistivity measurement, absorption or reflectivity measurement, andother techniques known to those skilled in the art.) Thereafter,responsive to detection of the condition indicative of non-uniformdoping of the crystalline material, laser power may be altered duringformation of subsurface laser damage patterns to provide laser emissionsat a first average power when forming subsurface laser damage in a firstdoping region, and to provide laser emissions at a second average powerwhen forming subsurface laser damage in a second doping region, whereinthe first and second average power levels differ from one another.Alternatively, or additionally, depth for formation of subsurface laserdamage in the substrate (i.e., relative to an exposed surface of thesubstrate) when forming subsurface laser damage may be altered in one ofthe first doping region or the second doping region. In certainembodiments, a difference in laser focusing depth between first andsecond doping regions for formation of a single reduced thicknessportion of a substrate (e.g., one wafer) may be in a range of 1 to 15microns, or 1 to 10 microns, or 2 to 8 microns, or 4 to 6 microns.

In certain embodiments, a crystalline material substrate may exhibitlaser absorption levels that vary with respect to vertical position inthe substrate (e.g., within an ingot), particularly for intentionallydoped material. Laser absorption levels may also vary from substrate tosubstrate (e.g., from ingot to ingot). It is believed that such changesmay be attributable to doping changes. In certain embodiments, a loweraverage laser power (e.g., 3 W) may be used for formation of subsurfacelaser damage in a substrate region distal from a growth seed, and ahigher average laser power (e.g., 5.5 W) may be used for formation ofsubsurface laser damage in a substrate region proximal to a growth seed.

In certain embodiments, for initial setting of laser subsurface damageto the correct depth relative to a surface of crystalline materialsubstrate, an optical measurement of the depth of laser focus in thesemiconductor material may be performed (e.g., taking into accountsemiconductor material/air index of refraction changes), and the settingof laser damage (e.g., laser power, laser focus, and/or number of laserdamage formation passes) may be adjusted responsive to such measurementprior to scanning an entire surface of the substrate. In certainembodiments, an optical measurement of a depth of laser focus may beperformed once per ingot, or each time after a portion of an ingot isfractured and removed (i.e., before formation of subsurface laser damagepattern(s) for each substrate layer to be removed by subsequentfracturing).

In certain embodiments, a semiconductor material processing method asdisclosed herein may include some or all of the following items and/orsteps. A second carrier wafer may be attached to a bottom side of acrystalline material substrate (e.g., ingot). Thereafter, a top side ofthe crystalline material substrate may be ground or polished, such as toprovide an average surface roughness R_(a) of less than about 5nanometers to prepare the surface for transmitting laser energy. Laserdamage may then be imparted at a desired depth or depths within thecrystalline material substrate, with spacing and direction of laserdamage traces optionally being dependent on crystal orientation of thecrystalline material substrate. A first carrier may be bonded to a topside of the crystalline material substrate. An identification code orother information linked to the first carrier is associated with a waferto be derived from the crystalline material substrate. Alternatively,laser marking may be applied to the wafer (not the carrier) prior toseparation to facilitate traceability of the wafer during and afterfabrication. The crystalline material substrate is then fractured (usingone or more methods disclosed herein) along a subsurface laser damageregion to provide a portion of the semiconductor material substratebound to the first carrier, and a remainder of the crystalline materialsubstrate being bound to the second carrier. Both the removed portion ofthe semiconductor material substrate and the remainder of thesemiconductor material substrate are ground smooth and cleaned asnecessary to remove residual subsurface laser damage. The removedportion of the semiconductor material substrate may be separated fromthe carrier. Thereafter, the process may be repeated using the remainderof the semiconductor material substrate.

Whereas wire sawing of SiC wafers typically entails kerf losses of atleast about 250 microns per wafer, laser- and carrier-assistedseparation methods disclosed herein and applied to SiC may achieve kerflosses in a range of from 80 to 140 microns per wafer.

In certain embodiments, laser subsurface damage may be formed in acrystalline material substrate prior to bonding the substrate to a rigidcarrier. In certain embodiments, a rigid carrier that is transparent tolaser emissions of a desired wavelength may be bonded to a crystallinematerial substrate prior to subsurface laser damage formation. In suchan embodiment, laser emissions may optionally be transmitted through arigid carrier and into an interior of the crystalline materialsubstrate. Different carrier-substrate subsurface laser formationconfigurations are shown in FIGS. 10A-10D. FIG. 10A is a schematic viewof laser emissions 61 being focused through a surface of a baresubstrate 62 to form subsurface laser damage 63 within the substrate 62,whereby a rigid carrier may be affixed to the substrate 62 followingformation of the subsurface laser damage. FIG. 10B is a schematic viewof laser emissions 61 being focused through a surface of a substrate 62form subsurface laser damage 63 within the substrate 62, with thesubstrate 62 having previously been bonded using adhesive material 64 toa rigid carrier 66. FIG. 10C is a schematic view of laser emissions 61being focused through a rigid carrier 66 and adhesive 64 to formsubsurface laser damage 63 within a substrate 62 previously bonded tothe rigid carrier 66. In certain embodiments, a surface of the substrate62 distal from the rigid carrier 66 may include one or more epitaxiallayers and/or metallization layers, with the substrate 62 embodying anoperative electrical device prior to formation of the subsurface laserdamage 63. FIG. 10D is a schematic view of laser emissions 61 beingfocused through a rigid carrier 66 into a substrate 62 (without anintervening adhesive layer) to form subsurface laser damage 63 withinthe substrate 62 previously bonded (e.g., via anodic bonding or otheradhesiveless means) to the rigid carrier 66.

Interspersed Subsurface Laser Damage

In certain embodiments, subsurface laser damage may be formed incrystalline material by sequential formation of multiple interspersedlaser damage patterns, with each subsurface laser damage patternincluding a plurality of substantially parallel lines. In certainembodiments, each subsurface laser damage pattern may extend oversubstantially an entire length (e.g., perpendicular to a substrate flat)and include spaced-apart lines distributed over substantially an entirewidth, of a substrate of crystalline material. In certain embodiments,interspersed damage patterns may include sequentially formed first andsecond, or first through third, or first through fourth, subsurfacelaser damage patterns, with each subsurface laser damage patternincluding multiple parallel lines. It is believed that sequentiallyforming multiple subsurface laser damage patterns in an interspersedfashion (e.g., forming a first subsurface damage pattern, then forming asecond subsurface damage pattern, then forming any subsequent subsurfacedamage pattern(s), with various lines of each damage pattern distributedamong the other damage patterns) is preferable to forming the sametraces without interspersing to promote ease of fracturing of thecrystalline material along or adjacent to a subsurface laser damageregion. Without wishing to be bound by any specific theory as to reasonsfor improved fracturing results obtained by interspersing of subsurfacelaser damage patterns in a crystalline material, it is believed thatsequential formation of interspersed subsurface laser damage patternsmay preserve a greater degree of internal stress within thesemiconductor material to facilitate lateral propagation of cracksemanating from different subsurface laser damage lines.

In certain embodiments, a first subsurface laser damage pattern in acrystalline material includes a first plurality of parallel lines and afirst plurality of cracks in the interior of the crystalline materialpropagating laterally outward (e.g., predominantly or substantiallyalong the c-plane) from lines of the first plurality of substantiallyparallel lines, wherein cracks emanating from each line arenon-connecting with cracks emanating from each adjacent line. In certainembodiments, a second subsurface laser damage pattern including a secondplurality of parallel lines is formed in the crystalline material afterformation of the first subsurface laser damage pattern, wherein thesecond subsurface laser damage pattern includes a second plurality ofcracks in the interior of the crystalline material propagating laterallyoutward from lines of the second plurality of substantially parallellines, and at least some cracks of the second plurality of cracksconnect with cracks emanating from two adjacent lines of the firstplurality of lines (e.g., to form continuous cracks).

In certain embodiments, first, second, and third subsurface laser damagepatterns are sequentially formed in a crystalline material, with eachsubsurface laser damage pattern including multiple parallel lines, andwith lines of each subsurface laser damage pattern being distributedamong lines of each other subsurface laser damage pattern. In certainembodiments, the first subsurface laser damage pattern comprises a firstplurality of cracks in the interior of the crystalline materialpropagating laterally outward from lines of the first plurality ofsubstantially parallel lines; the second subsurface laser damage patterncomprises a second plurality of cracks in the interior of thecrystalline material propagating laterally outward from lines of thesecond plurality of substantially parallel lines, with the secondplurality of cracks being non-connecting with the first plurality ofcracks; and the third subsurface laser damage pattern comprises a thirdplurality of cracks in the interior of the crystalline materialpropagating laterally outward from lines of the third plurality ofsubstantially parallel lines. In such an embodiment, at least somecracks of the third plurality of cracks connect with (i) at least somecracks of the first plurality of cracks and (ii) at least some cracks ofthe second plurality of cracks (e.g., to form continuous cracks). Incertain embodiments, a fourth subsurface laser damage pattern may beformed after the first through third subsurface laser damage patterns,with the fourth subsurface laser damage pattern serving to furtherconnect cracks emanating from any two or more of the first, second, orthird lines. In certain embodiments, three, four, five, or moreinterspersed patterns of subsurface laser damage may be provided.

In certain embodiments, one or more portions of a substrate may includeinterspersed subsurface laser damage patterns, while other portions of asubstrate may include non-interspersed laser damage patters. In certainembodiments, different interspersing patterns of subsurface laser damagemay be provided on the same substrate. For example, an interspersingpattern of subsurface laser damage on a single substrate may includefive damage lines in a first region, four damage lines in a secondregion, three damage lines in a third region, two damage lines in afourth region, one damage lines in a fifth region (i.e., withoutinterspersing), zero damage patterns in a sixth region, or anycombination of two or three of the foregoing, optionally wherein each ofthe foregoing regions has substantially the same unit area. In certainembodiments, a regular (e.g., regularly repeating) pattern ofinterspersed damage lines may exist in at least one region of thesubstrate, and an irregular (e.g., lacking regular repeat) pattern ofinterspersed damage lines or non-interspersed damage lines may exist inat least one other region of the substrate.

FIG. 11A provides a top plan view of a crystalline material substrate 70including interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment. The first, second,and third subsurface damage patterns separately include first, second,and third pluralities of parallel lines 71, 72, 73, respectively, thatextend perpendicular to a primary substrate flat 78 (and perpendicularto the [1120] direction). The three laser damage patterns in combinationforming multiple three-line groups 74 that are separated from oneanother by an inter-group spacing 75 that exceeds a spacing 76, 77between adjacent lines in each three-line group 74. For clarity, cracksformed by the first, second, and third pluralities of parallel lines 71,72, 73 are not illustrated in FIG. 11A. In certain embodiments, thefirst plurality of parallel lines 71 is formed in a first pass, thesecond plurality of parallel lines 72 is formed in a second pass, andthe third plurality of parallel lines 73 is formed in a third pass. Thethird pass may serve to connect cracks originally emanating from any ofthe first and/or second parallel lines 71, 72.

With continued reference to FIG. 11A, in one embodiment the firstplurality of parallel lines 71 may be formed with a 500 micron pitch(i.e., spacing between lines), and the second plurality of parallellines 72 may be formed with a 500 micron pitch and an offset of 250microns relative to the first plurality of parallel lines 71.Thereafter, the third plurality of parallel lines 73 may be formed witha 500 micron pitch, and an offset of 125 microns relative to the firstplurality of parallel lines 71. This arrangement creates multiplethree-line groups 74 that are separated from each other three-line groupby a 250 micron gap, with adjacent lines within each three-line groupbeing separated from each other by a gap of 125 microns.

The inventors have found that the order of the three-pass laser damageformation process described in connection with FIG. 11A is important. Ifthe order of passes is changed to sequentially form the first, third,and second pluralities of subsurface laser damage lines, then higherlaser power is required to complete cracking across the 250 microninter-group spacing 75. It is believed that this may be attributable tothe cracking that occurs between the 125 micron spaced lines on thesecond pass when using the original (first, second, third pass)sequential order, in which cracks formed in the third pass aresufficiently sized to just connect cracks emanating from the secondsubsurface damage lines across the second 125 micron gap 77. When theorder of the passes is first, third, second, then cracking across theinter-group spacing 75 is not observed unless laser power is increased,but laser power increases typically increase kerf loss. Thus, accordingto certain embodiments in which the order of passes is first, second,third, it may be desirable for cracks formed in the first and secondpasses not to connect with one another, and then for cracks formed inthe third pass to create connected cracks across both the 125 microngaps 76, 77 and the 250 micron inter-group spacing 75.

In certain embodiments, boundaries of each three-line group 74 may beconsidered to bound a damage-bearing area of the substrate 70, and thedamage-bearing area of each three-line group 74 is spaced apart from adamage-bearing area of each other three-line group (i.e., by theinter-group spacing 75). Notably, as will be shown in FIG. 11D, thecracks formed by subsurface laser damage may propagate between adjacentthree-line groups 74 across the inter-group spacing 75.

FIGS. 11B-11D illustrate fabrication of the crystalline materialsubstrate 70 of FIG. 11A. FIG. 11B illustrates the substrate 70following formation of a first plurality of subsurface laser damagelines 71 (perpendicular to a flat 78 of the substrate 70) having a pitch(or inter-line spacing) 71B and that form a first subsurface laserdamage pattern 71A. Cracks 71C propagate laterally outward from thefirst plurality of subsurface laser damage lines 71, but cracksemanating from different subsurface laser damage lines 71 do not connectwith one another.

FIG. 11B illustrates the substrate 70 following formation of a secondplurality of subsurface laser damage lines 72 (perpendicular to a flat78 of the substrate 70) having a pitch (or inter-line spacing) 72B andthat form a second subsurface laser damage pattern 72A. Cracks 72Cpropagate laterally outward from the second plurality of subsurfacelaser damage lines 71, but cracks emanating from different subsurfacelaser damage lines 71 do not connect with one another.

FIG. 11C illustrates the substrate 70 following formation of a thirdplurality of subsurface laser damage lines 73 (perpendicular to a flat78 of the substrate 70) having a pitch (or inter-line spacing) 73B andthat form a third subsurface laser damage pattern 73A. Cracks 73Cpropagate laterally outward from the third plurality of subsurface laserdamage lines 73, with such cracks 73C being sufficient to connect cracks71C, 72C formed by the first and second subsurface laser damage lines71, 72. As shown, connection of cracks between the first, second, andthird pluralities of subsurface damage lines is also sufficient to causecracks to further propagate and connect across the inter-group spacing75.

In certain embodiments, a third laser pass that forms the thirdsubsurface damage pattern is performed at a higher laser power levelthan the first two passes, to assist in extending cracks to connectacross the inter-group spacing 75, which is wider than the spacing 76,77 between lines within each three-line group 74. The inventors havefound that increasing laser power during the third pass sufficient toconnect not only cracks between laser subsurface damage line 125 umapart, but also between laser subsurface damage lines positioned 250 umapart (such as shown in FIG. 11D). This yields a roughly 25% toolthroughput increase with a small penalty in kerf loss (e.g.,approximately 110 um kerf loss instead of 100 um).

In certain embodiments, all laser subsurface damage lines may benon-perpendicular to a primary substrate flat (and to the [1120]direction), within a range of from about 1 degree to 5 degrees fromperpendicular. For example, FIG. 12 is top plan schematic view of acrystalline material substrate 80 including a substrate flat 88 andfirst, second, and third pluralities of substantially parallelsubsurface laser damage lines 81-83 that are interspersed orinterspersed among one another to form first through third subsurfacelaser damage patterns. Each plurality of substantially parallelsubsurface laser damage lines 81-83 deviates three degrees relative toperpendicular to a primary substrate flat (and to the [1120] direction),with the laser damage patterns in combination forming multiplethree-line groups 89 that are separated from one another by aninter-group spacing 85 that exceeds the spacing (or gaps) 86, 87 betweenadjacent lines in each three-line group 89. In one embodiment the firstplurality of parallel lines 81 may be formed with a 500 micron pitch(i.e., spacing between lines), and the second plurality of parallellines 82 may be formed with a 500 micron pitch and an offset of 250microns relative to the first plurality of parallel lines 81.Thereafter, the third plurality of parallel lines 83 may be formed witha 500 micron pitch, and an offset of 125 microns relative to the firstplurality of parallel lines 81. This arrangement creates multiplethree-line groups 89 that are separated from each other three-line groupby a 250 micron gap, with adjacent lines within each three-line groupbeing separated from each other by a gap of 125 microns. As shown, theparallel subsurface laser damage lines 81-83 of each group are parallelto one another.

FIG. 13 is top plan schematic view of a crystalline material substrate90 including a substrate flat 98 and first through fourth pluralities ofsubstantially parallel subsurface laser damage lines 91-94 that areinterspersed or interleaved among one another to form first throughfourth subsurface laser damage patterns with all lines being parallel toone another and perpendicular to the substrate flat 98 (and to the[1120] direction). In certain embodiments, the first through fourthpluralities of subsurface laser damage lines 91-94 may each includelines having a pitch of 500 nm, wherein the second plurality of lines 92are offset 250 microns from the first plurality of lines 91, the thirdplurality of lines is offset 125 microns from the first plurality oflines 91, and the fourth plurality of lines is offset 375 microns fromthe first plurality of lines 91. The net result is that a 125 micron gapis provided between each line of the first through fourth pluralities oflines 91-94. A four-line repeat group 95 is composed of the firstthrough fourth lines 91-94.

An alternative method for forming a crystalline material substratesimilar to the substrate 90 shown in FIG. 13 involves use of four passesof laser subsurface damage formation, with each pass forming lineshaving a 500 micron pitch. Following a first pass, lines formed by asecond pass are offset 125 microns from lines of the first pass, thenlines of the third pass are offset 250 microns from lines of the firstpass, then lines of the fourth pass are offset 375 microns from lines ofthe first pass.

FIG. 14 is top plan schematic view of a crystalline material substrate100 including a substrate flat 108, and including interspersed firstthrough third pluralities of subsurface laser damage lines 101-103forming first, second, and third subsurface laser damage patterns. Thefirst and second pluralities of lines 101, 102 are each parallel to oneanother and perpendicular to the primary substrate flat 108 (and to the[1120] direction), while the third plurality of lines 103 isnon-parallel to the first and second pluralities of lines 101, 102(e.g., with an angular difference in a range of from 1 to 5 degrees) butdoes not cross any of the first and second lines 101, 102 within thesubstrate 100. In certain embodiments, the first and second pluralitiesof parallel lines 101, 102 are formed first, and then the thirdplurality of parallel lines 103 is formed thereafter. In certainembodiments, the first and second pluralities of parallel lines 101, 102each have a 500 micron pitch, with the second plurality of parallellines 102 being offset 250 microns relative to the first plurality ofparallel lines 101. A multi-line repeat group 104 is composed of thefirst through third lines 101-103.

Although subsurface laser damage lines in FIG. 14 are non-crossing, incertain embodiments one or more subsurface laser damage lines (e.g.,formed in a subsequent laser damage formation pass) may cross one ormore other subsurface damage lines (e.g., formed in a prior or initiallaser damage formation pass). In certain embodiments, relative anglesbetween subsurface laser damage lines that cross may be in a range of 4to 30 degrees, or 5 to 20 degrees, or 5 to 15 degrees, or 5 to 10degrees.

FIG. 15 is top plan schematic view of a crystalline material substrate110 including a substrate flat 118, and including interspersed firstthrough third pluralities of subsurface laser damage lines 111-113 thatform first through third subsurface laser damage patterns. The first andsecond pluralities of lines 111, 112 are each parallel to one anotherand non-perpendicular to the primary substrate flat 108 (e.g., with anangular difference in a range of from 1 to 5 degrees), while the thirdplurality of lines 113 is perpendicular to the primary substrate flat118 but at least some (or all) lines thereof do not cross lines of thefirst and second groups of lines 111, 112 within the substrate 110. Incertain embodiments, the first and second pluralities of parallel lines111, 112 each have a 510 micron pitch, with the second plurality ofparallel lines 112 being offset 250 microns relative to the firstplurality of parallel lines 111. A three-line repeat group 114 iscomposed of the first through third lines 111-113.

FIG. 16 is a top plan schematic view of a crystalline material substrateincluding interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment in which all laserdamage lines are parallel to one another, and inter-group spacing oflaser damage lines is not uniform over at least portions of thesubstrate.

FIG. 17 is a top plan schematic view of a crystalline material substrateincluding interspersed first, second, and third subsurface laser damagepatterns defined therein according to one embodiment in which all laserdamage lines are parallel to one another, and laser damage lines exhibitvariation in intra-group spacing, inter-group spacing, and groupcomposition.

FIG. 18 is a top plan schematic view of a crystalline material substrateincluding sequentially formed first, second, and third subsurface laserdamage patterns defined therein according to one embodiment in whichfirst and second groups of laser damage lines are parallel to oneanother, while a third group of laser damage lines are non-parallel toand cross the first and second groups of laser damage lines.

FIG. 19 is a top plan schematic view of a crystalline material substrateincluding sequentially formed first, second, and third subsurface laserdamage patterns in which each group of laser damage lines includesparallel lines, and each group of laser damage lines is non-parallel toeach other group of laser damage lines. Although FIGS. 11A to 19illustrate embodiments including three or four pluralities of subsurfacelaser damage lines, it is to be appreciated that any suitable number ofsubsurface laser damage line groups may be provided. For example, incertain embodiments, first and second pluralities of subsurface laserdamage lines may be interspersed in the absence of third and/or fourthpluralities of subsurface laser damage lines. In certain embodiments,first and second pluralities of subsurface laser damage lines may beformed in first and second passes, respectively, with each plurality oflaser damage lines having a 250 micron pitch, and with the secondplurality of laser damage lines being offset by 125 microns relative tothe first plurality of laser damage lines.

In certain embodiments, subsurface laser damage is distributed amongmultiple non-overlapping areas of crystalline material by forming afirst group of subsurface laser damage sites in non-overlapping firstand second areas of the crystalline material, followed by formation of asecond group of subsurface laser damage sites in the first and secondareas, wherein at least some (or all) sites of the second group ofsubsurface laser damage sites do not cross sites of the first group ofsubsurface laser damage sites is formed in the non-overlapping areas.One or more additional groups of subsurface laser damage sites may beformed thereafter, and distributed among the same non-overlapping firstand second areas of the crystalline material. Although first and secondareas have been described, it is to be appreciated that any suitablenumber of non-overlapping areas may be defined (e.g., three, four, five,six, or more areas). In certain embodiments, such areas may not onlylack any overlap, but may also be spaced apart from one another (e.g.,spaced apart laterally) in a non-contacting relationship.

FIG. 20A is a top plan view of a crystalline material substrate 150illustrating non-overlapping first, second, and third areas 150A-150C inwhich laser damage regions may be formed. Although shading has beenadded to the first and third areas 150A, 150C for illustration purposesto emphasize boundaries between the first through third areas 150A-150C,it is to be appreciated that an actual crystalline material substrate150 would typically be uniform in color. Each area 150A-150C contacts aportion of a primary flat 150′ of the substrate 150. While three areas150A-150C are shown in FIGS. 20A-20D, any suitable number of areas arecontemplated, such as two, three, four, five, six, or more, and suchareas may be arranged in any suitable conformation such as in aone-dimensional array, in a two-dimensional array, in sectors (e.g.,wedge-shaped sectors) extending from a center point, etc.

FIG. 20B is a top plan view of the crystalline material substrate 150 ofFIG. 20A following formation of a first plurality of subsurface laserdamage regions 151 in the first through third areas 150A-150C. As shown,the laser damage regions 151 are provided as substantially parallellines that are substantially perpendicular to a primary flat 150′ of thesubstrate 150. Multiple laser damage regions 151 are provided in each ofthe first through third areas 150A-150C. Although not illustrated inFIG. 20B, it is to be appreciated that laterally extending cracks (suchas shown in FIG. 11B) may emanate from the laser damage regions 151, butpreferably not connect between adjacent laser damage regions 151. Incertain embodiments, subsurface laser damage regions 151 of theplurality of subsurface laser damage regions 151 may be formed in thefirst area 150A, then in the second area 150B, and finally in the thirdarea 150C.

FIG. 20C is a top plan view of the crystalline material substrate 150 ofFIG. 20B following formation of a second plurality of subsurface laserdamage regions 152 in the first through third areas 150A-150C. As shown,the laser damage regions 152 of the second plurality of subsurface laserdamage regions 152 are provided as substantially parallel lines that aresubstantially perpendicular to the primary flat 150′, and multiple laserdamage regions 152 are provided in each of the first through third areas150A-150C. Additionally, each laser damage region 152 of the secondplurality of subsurface laser damage regions 152 is substantiallyparallel to the first plurality of subsurface laser damage regions 151.Although not shown in FIG. 20C, it is to be appreciated that laterallyextending cracks may emanate from each laser damage region 151, 152, butsuch cracks preferably do not connect between adjacent laser damageregions 151, 152. In certain embodiments, subsurface laser damageregions 152 of the plurality of subsurface laser damage regions 152 maybe formed in same sequence as the first subsurface laser damage regions151 (e.g., the subsurface laser damage regions 152 may be formed in thefirst area 150A, then in the second area 150B, and finally in the thirdarea 150C). In this manner, laser damage regions 152 of the secondplurality of subsurface laser damage regions 152 are interspersed amonglaser damage regions 151 of the first plurality of subsurface laserdamage regions 151.

FIG. 20D is a top plan view of the crystalline material substrate ofFIG. 20C following formation of a third plurality of subsurface laserdamage regions 153 in the first through third areas 150A-150C. As shown,the laser damage regions 153 of the third plurality of subsurface laserdamage regions 153 are provided as substantially parallel lines that aresubstantially perpendicular to the primary flat 150′, and multiple laserdamage regions 153 of the third plurality of subsurface laser damageregions 153 are provided in each of the first through third areas150A-150C. Each laser damage region 153 of the third plurality ofsubsurface laser damage regions 153 may be substantially parallel to thefirst and second pluralities of subsurface laser damage regions 151,152. Subsurface laser damage patterns provided by the first throughthird subsurface laser damage regions 151-153 form a plurality ofthree-line groups 154 that are spaced apart from one another by aninter-group spacing 154′ that exceeds a spacing between adjacent laserdamage regions 151-153 in each three-line group 154. Although not shownin FIG. 20C, it is to be appreciated that laterally extending cracks mayemanate from each laser damage region 151-153, with the cracks extendinglaterally among all laser damage region 151-153 (such as shown in FIG.11D) to facilitate subsequent fracture of an upper portion of thesubstrate 150 from a remainder of the substrate 150. In certainembodiments, subsurface laser damage regions 153 of the plurality ofsubsurface laser damage regions 152 may be formed in same sequence asthe first and second subsurface laser damage regions 151, 152 (e.g., thesubsurface laser damage regions 153 may be formed in the first area150A, then in the second area 1508, and finally in the third area 150C).In this manner, laser damage regions 153 of the third plurality ofsubsurface laser damage regions 153 are interspersed among laser damageregions 151, 152 of the first and second pluralities of subsurface laserdamage regions 151, 152.

Parallel Processing and/or Laser Beam Splitting

In certain embodiments, multiple regions of one substrate may beprocessed simultaneously to form subsurface laser damage in multiplesubstrate regions, and/or multiple substrates may be arranged within asingle tool for simultaneous or substantially simultaneous laserprocessing, to enhance tool throughput. In certain embodiments, anoutput beam of one laser may be split into multiple beams using one ormore beam splitters, individual beams of the beams may either besupplied to different substrates or different areas of a singlesubstrate, to form subsurface laser damage therein utilizing methodsdisclosed herein. In certain embodiments, multiple lasers may be used tosimultaneously supply beams to multiple substrates or multiple areas ofa single substrate, to form subsurface laser damage therein utilizingmethods disclosed herein.

FIG. 21 is a top plan schematic view of a holder 163 of a laserprocessing apparatus arranged to hold four substrates 155A-155D in whichsubsurface laser damage may be formed with one or more lasers. As shown,each substrate 155A-155D includes subsurface laser damage patternsdefined therein, with such patterns including first, second, and thirdpluralities of substantially parallel lines 156-158. The three laserdamage patterns in combination forming multiple three-line groups 156that are separated from one another by an inter-group spacing 160 thatexceeds a spacing 161, 162 between adjacent lines in each three-linegroup 159. In certain embodiments, laser damage patterns may be formedin the first and third substrates 155A, 155C with a first laser or afirst split laser beam portion, and laser damage patterns may be formedin the second and fourth substrates 155B, 155D with a second laser orsecond split laser beam portion. In certain embodiments, the holder 163bearing the substrates 155A-155D is configured to move (e.g., in two (x,y) lateral directions) while one or more lasers and/or focusing opticsthereof are restrained from lateral movement (but may be subject tovertical (z-direction) movement).

FIG. 22A is a top plan schematic view of a single substrate 164 beingprocessed with a laser beam split into multiple portions tosimultaneously form subsurface laser damage regions according to a firstsubsurface laser damage pattern in multiple areas of the substrate 164.As shown, the substrate 164 includes multiple areas 164A-164C (e.g.,resembling the areas 150A-150C depicted in FIGS. 20A-20C). An initiallaser damage formation step includes impinging two split laser beamportions to simultaneously form laser damage regions 165′ in the firstand second areas 164A, 164B. The substrate 164 may be laterally indexedrelative to a laser (e.g., in a direction opposite the rightwardarrows), and a subsequent laser damage formation step includes impingingtwo split laser beam portions to simultaneously form laser damageregions 165″ in the first and second areas 164A, 164B. This process isrepeated to form additional laser damage regions 165′″, 165″″ in thefirst and second areas 164A, 164B, and eventually to cover the first,second, and third areas 164A-164C to form a first subsurface laserdamage pattern. Thereafter, the process may be repeated to form secondand third subsurface laser damage patterns, respectively, that areinterspersed with the first subsurface laser damage pattern. The firstand second split laser beam portions may be used to form a subsurfacelaser damage patterns distributed over an entirety of the substrate 164in half the time that the patterns could be formed with single,undivided laser beam. FIG. 22B is a top plan schematic view of twosubstrates 166A, 166B supported by a holder 168 and being processed witha laser beam split into two portions to simultaneously form subsurfacelaser damage according to at least one subsurface laser damage patternin both substrates 166A, 166B. An initial laser damage formation stepincludes impinging two split laser beam portions to simultaneously formlaser damage regions 167′ in the first and second substrates 166A, 166B.The holder bearing the substrates 166A, 166B may be laterally indexedrelative to a laser (e.g., in a direction opposite the rightwardarrows), and a subsequent laser damage formation step includes impingingtwo split laser beam portions to simultaneously form laser damageregions 167″ in the first and second substrates 166A, 166B. This processis repeated to form additional laser damage regions 167′″, 167″″ in thefirst and second substrates 166A, 166B, and eventually to cover thefirst and second substrates 166A, 166B to form a first subsurface laserdamage patterns therein. Thereafter, the process may be repeated to formsecond and third subsurface laser damage patterns in the substrates166A, 166B, respectively, that are interspersed with the firstsubsurface laser damage pattern.

Formation of Overlapping Subsurface Laser Damage at Different Depths

In certain embodiments, initial subsurface laser damage centered at afirst depth may be formed within an interior of a crystalline materialsubstrate, and additional subsurface laser damage centered at a seconddepth may be formed within the interior of the substrate, wherein theadditional subsurface laser damage is substantially registered with theinitial subsurface laser damage, and a vertical extent of at least aportion of the additional subsurface laser damage overlaps with avertical extent of at least a portion of the initial laser damage.Restated, one or more subsequent passes configured to impart laserdamage at a different depth may be added on top of one or more priorpasses to provide subsurface laser damage with an overlapping verticalextent. In certain embodiments, addition of overlapping subsurfacedamage may be performed responsive to a determination (e.g., by opticalanalysis) prior to fracturing that one or more prior subsurface laserdamage formation steps was incomplete. In certain embodiments, adifference in laser focusing depth between first and second laser damagepatterns for formation of a single reduced thickness portion of asubstrate (e.g., one wafer) may be in a range of 1 to 10 microns, or 2to 8 microns, or 2 to 6 microns. Formation of overlapping subsurfacelaser damage at different depths may be performed in conjunction withany other method steps herein, including (but not limited to) formationof multiple interspersed subsurface laser damage patterns.

FIG. 23A is a cross-sectional schematic view of a crystalline materialsubstrate 170 including a first subsurface laser damage pattern 173centered at a first depth relative to a first surface 171 of thesubstrate 1770, with the subsurface damage pattern 173 produced byfocused emissions of a laser 179. The first subsurface laser damagepattern 173 has a vertical extent 174 that remains within an interior ofthe substrate 170 between the first surface 171 and an opposing secondsurface 172. FIG. 23B is a cross-sectional schematic view of thesubstrate of FIG. 23A following formation of a second subsurface laserdamage pattern 175 centered at a second depth and registered with thefirst subsurface laser damage pattern 173, wherein a vertical extent 176of the second damage pattern 175 overlaps with a vertical extent 174 ofthe first damage pattern 173 in a damage overlap region 177. In certainembodiments, subsequent fracturing of the crystalline material 170 maybe performed along or through the damage overlap region 177.

Formation of Non-Overlapping Subsurface Laser Damage at Different Depths

In certain embodiments, subsurface laser damage lines may be formed atdifferent depths in a substrate without being registered with other(e.g., previously formed) subsurface laser damage lines and/or withoutvertical extents of initial and subsequent laser damage beingoverlapping in character. In certain embodiments, an interspersedpattern of subsurface laser damage may include groups of laser lineswherein different groups are focused at different depths relative to asurface of a substrate. In certain embodiments, a focusing depth ofemissions of a laser within the interior of the substrate differs amongdifferent groups of laser lines (e.g., at least two different groups offirst and second groups, first through third groups, first throughfourth groups, etc.) by a distance in a range from about 2 microns toabout 5 microns (i.e., about 2 μm to about 5 μm).

Laser Tool Calibration

One of the primary drivers of kerf loss is subsurface laser damage belowthe primary fracture region on the ingot side. In general, an increasein subsurface laser damage increases kerf loss. One potential cause ofincreased subsurface laser damage is a failure to adequately compensatefor the optical characteristics of the crystalline material.

In certain embodiments, laser calibration may be performed each time acrystalline material substrate (e.g., ingot) is supplied to the lasertool, prior to formation of subsurface laser damage therein. Suchcalibration may utilize variable height adjustment to attainment of aninitial state in a best focus point of the laser beam is formed an uppersurface of the crystalline material substrate, followed by adjusting theaperture or correction collar of the laser tool corresponding to adesired depth of formation of subsurface laser damage in the crystallinematerial according to a subsequent state.

Wafer Photographs Showing Doping Region (a/k/a Doping Ring)

FIG. 24A is a perspective view photograph of a SiC wafer 180 followingseparation from a carrier (i.e., the thermoplastic glue-bonded sapphirecarrier 181 shown in FIG. 24B) using a thermally-induced fracture methoddescribed herein. Both the wafer 180 and the carrier 181 have a diameterof 150 mm. No wafer breakage was observed following thermally inducedfracture. FIG. 24C is a partially tone-reversed version of the SiC waferphotograph of FIG. 24A to emphasize contrast between a central dopingring 182 and an annular outer portion 183 of the SiC wafer 180. FIG. 24Dshows the image of FIG. 24C annotated with a dashed-line oval to denotea boundary between the central doping ring 182 and the annular outerportion 183 of the SiC wafer 180. The doping ring 182 represents aregion of increased doping relative to the annular outer portion 183 ofthe SiC wafer. Since doped semiconductor material such as SiC exhibitsincreased absorption of IR wavelengths, higher laser power may bebeneficial when seeking to form subsurface laser damage in the SiC waferin the doping ring 182 as compared to the annular outer portion 183. Incertain embodiments, presence of a condition indicative of non-uniformdoping of a crystalline material across at least a portion of a surfaceof the substrate may be detected, such as by detecting a change in lightreflection or absorption by optical means to determine presence of atleast one first doping region and at least one second doping region(e.g., the doping ring 182 and the annular outer portion 183).Thereafter, responsive to detection of the condition indicative ofnon-uniform doping of the crystalline material, laser power may bealtered during formation of subsurface laser damage patterns to providelaser emissions at a first average power when forming subsurface laserdamage in a first doping region (e.g., the doping ring 182), and toprovide laser emissions at a second average power when formingsubsurface laser damage in a second doping region (e.g., the annularouter portion 183), wherein the first and second average power levelsdiffer from one another.

Schematic Views of Ingots Exhibiting Doping Ring

FIG. 25 is a side cross-sectional schematic view of a SiC ingot 184grown on a seed crystal 185, showing a generally cylinder-shaped dopingregion 187 extending upward from the seed crystal 185 (at a first orbottom surface 185′ of the ingot 184) through the entire thickness ofthe ingot 184 along a central portion thereof, wherein the doping region187 is present at a second or top surface 186′ of the ingot 184. Thedoping region 187 is laterally surrounded by an undoped (e.g.,lower-doped or unintentionally doped) region 186 that is generallyannular in shape. A thin cross-sectional portion 189 of the ingot 184taken between the first and second surfaces 185′, 186′ may define awafer 189A, as shown in FIG. 26. The wafer 189A includes a centraldoping region 187 and a generally annular-shaped undoped region 186,which is bounded in part by a primary flat 189′. In certain embodiments,the wafer 189A may be produced from the ingot 184 using a laser-assistedparting method as described herein.

Although FIG. 26 shows the size (e.g., width or diameter) of the dopingregion 187 as being substantially constant throughout the thickness ofthe ingot 184, the inventors have observed that the size of a dopingregion can vary with vertical position in an ingot (e.g., typicallybeing larger in width or diameter closer to a seed crystal, and smallerwith increasing distance away from the seed crystal). It has also beenobserved that the magnitude of doping within the doping region can varywith vertical position in an ingot.

FIG. 27 is a side cross-sectional schematic view of a SiC ingot 184Agrown on a seed crystal 185A, showing a frustoconically shaped dopingregion 187A extending upward from the seed crystal 185A (at a first orbottom surface 185A′ of the ingot 184A) through the entire thickness ofthe ingot 184A along a central portion thereof. As shown, the dopingregion 187A is present at a second or top surface 186A′ of the ingot184A, but the doping region 187A is smaller in width or diameter at thesecond surface 186A′ than at the first surface 185A′. The doping region187A is laterally surrounded by an undoped (e.g., lower-doped orunintentionally doped) region 186A that is generally annular in shape.In certain embodiments, the doping region 187A may have a width and adoping level that vary with vertical position relative to the seedcrystal 185A.

The inventors have also observed that lateral position and shape of adoping region can differ relative to the configuration shown in FIG. 27if a vicinal (e.g., offcut an angle non-parallel to c-plane) seedcrystal is used for growth of a SiC ingot. For example, if a vicinalseed crystal is used, then a doping region may be more oval than roundin shape, and/or may be offset laterally relative to a center of aningot.

FIG. 28 is a side cross-sectional schematic view of a SiC ingot 1846grown on a vicinal (e.g., offcut) seed crystal 1856, showing afrustoconically shaped doping region 1876 extending upward from the seedcrystal 1856 at a point offset from a center of seed crystal 1856 andupward through the entire thickness of the ingot 1846. As shown, thedoping region 1876 is present at a second or top surface 1866′ of theingot 1846, but the doping region 1876 is smaller in width or diameterat the second surface 1866′ than at the first surface 1856′. The dopingregion 1876 may have a generally oval shape when viewed from above. Thedoping region 1876 is laterally surrounded by an undoped (e.g.,lower-doped or unintentionally doped) region 1866. In certainembodiments, the doping region 1876 may have a shape, a width, and/or adoping level that varies with vertical position relative to the seedcrystal 1856.

Magnified Wafer Photographs

FIG. 29 is a perspective view photograph of a Si face of a SiC waferseparated from an ingot by a process involving formation of subsurfacelaser damage and subsequent separation, with an inset portion (upperright) depicting an intentionally separated fragment of the SiC waferincluding an edge depicted in subsequent scanning electron microscope(SEM) images.

FIG. 30A is a 45 times magnification SEM image, taken at a 15 degreetilt angle, of a portion of the SiC wafer fragment of FIG. 29, withsuperimposed arrows showing directions of the [1100] and [1120]crystallographic planes. Laser lines are perpendicular to the [1120]direction spaced at about 250 microns therebetween. FIG. 30B is a 1,300times magnification SEM image, taken at a 15 degree tilt angle, of aportion of the SiC wafer fragment of FIG. 29. FIG. 30C is a 350 timesmagnification SEM image, taken at a 15 degree tilt angle, of a portionof the SiC wafer fragment of FIG. 29. As shown in FIG. 30C, off-axiscleave planes roughly correlate with the laser spacing, but are notconsistent across the entire wafer surface. This may be attributable atleast on part to variation in laser line position on cleave planes. Inthis wafer, fracture was initiated at a polycrystalline inclusion.

FIG. 30D is a 100 times magnification SEM image taken at a 2 degree tiltangle, of a portion of the SiC wafer fragment of FIG. 29. FIG. 30E is a1,000 times magnification SEM image taken at a 2 degree tilt angle, of aportion of the SiC wafer fragment of FIG. 29. FIGS. 30D and 30E showthat laser damage is fairly shallow compared to surface features alongthe fracture region. Variability in the resulting fracture damage isvisible, particularly in a central portion of FIG. 30E.

FIG. 31A is a confocal laser scanning microscopy image of a small,central portion of the SiC wafer of FIG. 29, with superimposedcrosshairs marking positions of “trenches” formed by laser scanning.FIG. 31B is a surface profile plot of the portion of the SiC wafer ofFIG. 31A. With reference to FIG. 31B, variability in laser line positionrelative to SiC cleave planes is observable.

FIG. 32A is a confocal laser scanning microscopy image of a larger,top-proximate (as pictured) portion of the SiC wafer of FIG. 29, withsuperimposed crosshairs marking positions of “trenches” or lines formedby laser scanning. FIG. 32B is a surface profile plot of thetop-proximate portion of the SiC wafer of FIG. 32A. In FIG. 32B, a firstpair of lines corresponding to laser damage (represented as crosshairswithin oval 200) are separated by a depth of more than 30 microns, and asecond pair of lines corresponding to laser damage (represented ascrosshairs within oval 201) are separated by a depth of more than 20microns. An irregular spacing between laser lines is shown in FIGS. 32Aand 32B, wherein individual lines within the first pair of lines (withinoval 200) are closer to one another, and individual lines within thesecond pair of lines (within oval 201), are closer to one another thanother depicted laser damage lines.

FIG. 33A is a confocal laser scanning microscopy image of a larger,bottom-proximate (as pictured) portion of the SiC wafer of FIG. 29, withsuperimposed crosshairs marking positions of “trenches” formed by laserscanning. FIG. 33B is a surface profile plot of the bottom-proximateportion of the SiC wafer of FIG. 33A. FIG. 33B shows lateral distancevariation between adjacent pairs of laser damage lines, with one pairseparated by 334 microns, and another separated by 196 microns, but amaximum depth variation of 13 microns.

Fracturing of Substrate Following Formation of Subsurface Laser Damage

As discussed previously herein, subsurface laser damage may be formedwithin a crystalline material substrate to prepare the substrate forfracturing to remove at least one thin layer of crystalline material(e.g., a wafer) from the substrate. Although examples of specificfracturing techniques are described hereinafter (e.g., cooling aCTE-mismatched carrier joined to a substrate, impinging ultrasonic waveson a substrate, or imparting a bending moment on a carrier mounted tosubstrate), it is to be appreciated that various subsurface laser damageformation techniques described herein may be used within any suitablefracturing techniques, including fracturing techniques already known toone skilled in the art.

Fracturing by Cooling Rigid Carrier with Carrier/Substrate CTE Mismatch

FIGS. 34A-34F illustrate steps of a carrier-assisted method forfracturing a crystalline material according to one embodiment of thepresent disclosure, utilizing a rigid carrier having a greater CTE thanthe crystalline material joined to the crystalline material. FIG. 34A isside cross-sectional schematic view of a rigid carrier 202 having alayer of adhesive material 198 joined to a first surface 203 of therigid carrier 202, and having a second surface 204 that opposes thefirst surface 203.

FIG. 34B is a cross-sectional schematic view of an assembly 188including the rigid carrier 202 and adhesive material 198 of FIG. 34Ajoined to a crystalline material substrate 190 having a subsurface laserdamage region 196 therein. The rigid carrier 202 has a greater diameteror lateral extent than the substrate 190. The substrate 190 includes afirst surface 192 proximate to the adhesive material 198, and includesan opposing second surface 194, with the subsurface laser damage 196being closer to the first surface 192 than to the second surface 194.The adhesive material 198 extends between a first surface 192 of thecrystalline substrate 190 and the first surface 203 of the rigid carrier202. The adhesive material 198 may be cured according to therequirements of a selected bonding method (e.g., thermo-compressionadhesive bonding, compression-aided UV bonding, chemically reactivebonding, etc.). In certain embodiments, a second carrier (not shown) maybe bonded to the second surface 194 of the substrate 190, with thesecond carrier optionally being no wider than and/or CTE matched withthe substrate 190.

FIG. 34C is a cross-sectional schematic view of the assembly of FIG.34B, following positioning of the second surface 204 of the rigidcarrier 202 on a support surface 208 of cooling apparatus in the form ofa cooled chuck 206 configured to receive a cooling liquid. Contactbetween the rigid carrier 202 and the cooled chuck 206 causes heat to betransferred from the rigid carrier 202 to the cooled chuck 206. Duringthe cooling process, the rigid carrier 202 will laterally contract to agreater extent than the crystalline material substrate 190 due to agreater CTE of the carrier 202 than the substrate 190, such that thecarrier 202 exerting shear stress on the substrate 190. Due to thepresence of subsurface laser damage 196 near the adhesive layer 198 thatjoins the rigid carrier 202 to the substrate 190, the exertion of shearstress on the substrate 190 causes the crystalline material to fracturealong or proximate to the subsurface laser damage region 196.

In certain embodiments, the cooled chuck 206 has a smaller diameter thana diameter of the rigid carrier 202. Although the cooled chuck 206 maybe supplied with a cooling liquid, it is not necessary for the rigidcarrier 202 to reach the liquid nitrogen temperature (−160° C.) tosuccessfully complete thermal-induced fracture of the crystallinematerial substrate 190. Favorable separation results have been obtainedfor fracturing single crystal SiC material supported by a single crystalsapphire substrate using a cooled chuck maintained at −70° C. Suchtemperature can be maintained using various cooling liquids, such asliquid methanol (which remains flowable above its freezing point at −97°C.) received from a two-phase pumped evaporative cooling system.Favorable separation results have also been obtained by cooling acarrier, adhesive, and a substrate in a freezer maintained at −20° C.,wherein such temperature may be maintained using a single phaseevaporative cooling system. The ability to use a single phaseevaporative cooling system or a two-phase pumped evaporative coolingsystem rather than liquid nitrogen significantly reduces operatingcosts.

FIG. 34D is a cross-sectional schematic view of a remainder of thecrystalline material substrate 190A separated from a bonded assemblythat includes the rigid carrier 202, adhesive material 198, and aportion of the crystalline material 210 removed from the remainder ofthe substrate 190A, following fracture of the crystalline material alongthe subsurface laser damage region. The remainder of the crystallinematerial substrate 190A is bounded by a new first surface 193 (havingresidual laser damage 196A) that opposes the second surface 194.Correspondingly, the removed portion of crystalline material 210 isbounded by a new second surface 212 (having residual laser damage 1968)that opposes the first surface 192. Thereafter, the bonded assembly 215including the rigid carrier 202, the adhesive material 198, and theremoved portion of crystalline material 160 may be withdrawn from thecooled chuck 206.

FIG. 34E is a cross-sectional schematic view of the bonded assembly 215of FIG. 34D, following withdrawal from the liquid-cooled chuck 206.Maintaining the removed portion of crystalline material 210 attached tothe rigid carrier 202 beneficially provides mechanical support for theremoved portion of crystalline material 210 to permit one or moresurface processing steps (e.g., grinding, polishing, etc.) to beperformed on the new surface 212, to remove the residual laser damage1968 and achieve a desirable thickness of the crystalline material 210(e.g., via grinding, optionally followed by chemical mechanicalplanarization and/or polishing steps). In certain embodiments, laserdamage removal and thinning may include sequential grinding/polishingoperations, and any suitable polishing and cleaning steps to prepare thenew surface 212 for subsequent operations (e.g., surface implantation,laser marking (e.g., along a wafer flat), formation of epitaxial layers,metallization, etc.).

FIG. 34F is a cross-sectional schematic view of the removed portion ofthe crystalline material 210 supported by an upper surface 218 of aheated vacuum chuck 216, with the rigid carrier 202 and adhesivematerial 198 being laterally translated away from the removed portion ofcrystalline material 212 portion following elevated temperaturesoftening and release of the adhesive material 198. That is, the heatedvacuum chuck 216 may heat the adhesive material 198 to a sufficienttemperature to soften and/or flow, such that upon application of anexternal shear stress to the second surface 204 of the rigid carrier202, the rigid carrier 202 is permitted to laterally translate away fromthe removed portion of crystalline material 212 that is temporarily heldin place by the heated vacuum chuck 216. Thereafter, the heated vacuumchuck 216 may be deactivated, and the removed portion of crystallinematerial 212 embodies a freestanding material. If desired, any residuefrom the adhesive 198 may be removed and cleaned from the first surface203 of the rigid carrier 202, and the rigid carrier 202 optionally maybe re-used for another fracturing operation. The removed crystallinematerial can then be used as a growth substrate for deposition of one ormore epitaxial layers and conducting metal layers to from a device waferthen singulated to form discrete semiconductor devices.

Fracturing Induced by Ultrasonic Energy

Another method for effectuating fracture along a laser-inducedsubsurface damage zone of a crystalline material bonded to a rigidcarrier involves application of ultrasonic energy to the crystallinematerial while in the bonded state. FIG. 35 is a cross-sectionalschematic view of an assembly 188A including a crystalline material 190Ahaving subsurface laser damage 196A and bonded to a rigid carrier 202Ausing an intervening adhesive material 198A, with assembly 188A arrangedin a liquid bath 225 of an ultrasonic generator apparatus 220. Theapparatus 220 further includes a vessel 222 arranged in contact with anultrasonic generating element 224, with the vessel 222 containing theliquid bath 225. Presence of the rigid carrier 202A may reduce oreliminate breakage of the crystalline material 190A when subjected toultrasonic energy, particularly if residual stress remains between therigid carrier 202A and the crystalline material 190A prior to separation(e.g., due to a CTE mismatch). Such residual stress may reduce theamount of ultrasonic energy required to initiate fracture of thecrystalline material, thereby reducing the likelihood of materialbreakage.

Fracturing Induced by Mechanical Force

In certain embodiments, fracturing of a crystalline material bonded to arigid carrier may be promoted by (i) application of a mechanical force(e.g., optionally localized at one or more points) proximate to at leastone edge of the carrier. Such force may impart a bending moment in atleast a portion of the carrier, with such bending moment beingtransmitted to the subsurface laser damage region to initiate fracture.An exemplary embodiment is shown in FIGS. 36A-36C.

FIGS. 36A-36C are cross-sectional schematic views illustrating steps forfracturing a crystalline material substrate 236 having subsurface laserdamage 233 by application of a mechanical force proximate to one edge ofa carrier 238 to which the substrate 236 is bonded. The bonded assemblyincludes a crystalline material substrate 236 having a subsurface laserdamage region 233 being bonded between rigid carriers 238, 238′. Eachrigid carrier 238, 238′ includes a laterally protruding tab portion 239,239′ registered with a flat 235 of the substrate 236, providing a localincreased border region that defines a recess 231 into which a tool 219may be inserted. FIG. 36A illustrates a state prior to insertion of thetool 219 into the recess 191. FIG. 36B illustrates a state followinginsertion of the tool 219 into the recess, when the tool 216 is tiltedupward, thereby exerting a prying force in a direction tending topromote separation between the rigid carriers 238, 238′, therebyexerting a bending moment Mon at least one carrier 238. In certainembodiments, the substrate 236 comprises a material (e.g., 4H—SiC)having hexagonal crystal structure and the bending moment M is orientedwithin ±5 degrees of perpendicular to the [1120] direction (or,equivalently, within ±5 degrees of parallel to the [1-100] direction) ofthe hexagonal crystal structure. FIG. 36C illustrates a state followinginitial fracture of the crystalline substrate 236 along the subsurfacelaser damage region 233, whereby an upper portion 236 of the crystallinematerial remains bonded to the upper carrier 238, and a lower portion236B of the crystalline material remains bonded to the lower carrier238′, and the upper carrier 238 is tilted upward relative to the lowercarrier 238′. Such fracture yields a first bonded assembly 229A(including the upper carrier 238 and the upper portion 236A of thecrystalline material) separated from a second bonded assembly 229B(including the lower carrier 238′ and the lower portion 236B of thecrystalline material). In certain embodiments, mechanical force may beapplied proximate to opposing edges of a rigid carrier to which asubstrate is bonded to promote fracture of a crystalline material havingsubsurface laser damage that is bonded to the carrier.

It is noted that it is specifically contemplated to combine two or morefracturing techniques (e.g., CTE mismatch and ultrasonic inducedfracturing; or CTE mismatch and mechanical induced fracturing; orultrasonic induced and mechanical induced fracturing). In certainembodiments, liquid of an ultrasonic bath may be cooled either before orduring application of ultrasonic energy. Amount of mechanical force thatmay be required to complete fracture may be affected by CTE differentialbetween a substrate and a carrier. In certain embodiments, CTEdifferential and mechanical force may be combined. If a CTE differentialbetween a carrier and substrate is small or nonexistent (i.e., matchedCTE), then more mechanical force may be required to complete fracture.Conversely, if a CTE mismatch is large, then reduced mechanical force orno mechanical force may be required to complete fracture.

Device Wafer Splitting Process

In certain embodiments, a laser- and carrier-assisted separation methodmay be applied to a crystalline material after formation of at least oneepitaxial layer thereon (and optionally at least one metal layer) aspart of an operative semiconductor-based device. Such a device wafersplitting process is particularly advantageous for the ability toincrease yield (and reduce waste) of crystalline material bysignificantly reducing the need for grinding away substrate materialfollowing device formation.

FIGS. 37A-37O are cross-sectional schematic views illustrating steps ofa device wafer splitting process, according to which a thick wafer isfractured from a crystalline material, at least one epitaxial layer isgrown on the thick wafer, and the thick wafer is fractured to form afirst and second bonded assemblies each including a carrier and a thinwafer divided from the thick wafer, with the first bonded assemblyincluding the at least one epitaxial layer as part of an operativesemiconductor-based device.

FIG. 37A illustrates a crystalline material substrate 240 having a firstsurface 241 and subsurface laser damage 243 arranged at a depth relativeto the first surface. FIG. 37B illustrates the substrate 240 of FIG. 37Afollowing addition of adhesive material 244 over the first surface 241.FIG. 37C illustrates the items depicted in FIG. 37B following bonding ofa rigid carrier 246 to the substrate 240 using the adhesive material244. FIG. 37D illustrates the items of FIG. 37D following fracturing ofthe substrate 240 along the subsurface laser damage 243 (e.g., using oneor more methods disclosed herein), yielding a remainder of the substrate240 that is separated from a bonded assembly that includes the carrier246, the adhesive material 244, and a crystalline material portion(e.g., a thick wafer) 242 removed from the substrate 240. In certainembodiments, the thick wafer 242 may have a thickness in a range ofroughly 350 to 750 microns. Exposed surfaces 243A, 243B of the thickwafer 242 and the remainder of the substrate 240, respectively mayexhibit surface irregularities that may be reduced by surface processingsteps such as grinding, CMP, polishing, etc. FIG. 37E shows the thickwafer 242 following de-bonding and removal from the carrier 246, withthe thick wafer 242 including a perpendicular edge profile.Perpendicular edges of wafers fracture readily, producing unacceptableedge chips and particles during wafer handling. To reduce the risk ofbreakage, a wafer edge may be edge ground to produce a non-perpendicularwafer edge having a beveled or rounded edge. FIG. 37F shows the thickwafer 242 supported between opposing upper and lower gripping portions248A, 248B of a turntable proximate to a rotary profile grinding tool249 having a concave cutting surface 249A (e.g., impregnated withdiamond particles) configured to impart a rounded edge profile 247 tothe thick wafer 242. FIG. 37G shows the thick wafer 242 after edgegrinding (also known as edge profiling), with the thick wafer includinga rounded edge 247 providing a boundary between first and second wafersurfaces 251, 252.

FIG. 37H shows the thick wafer 242 of FIG. 37G following deposition ofone or more epitaxial layers 253 on or over the first surface 251 of thethick wafer 251. Due to the incompatibility of adhesives with the hightemperatures inherent to epitaxy, carrier shown in FIG. 37D is notpresent. FIG. 37I shows the structure FIG. 37H, following formation ofconductive (e.g., metal) contacts 254 over the epitaxial layers 253 toform at least one operative semiconductor device, with the thick wafer242 still having a rounded edge 247. Conventionally, grinding would beperformed on the second surface 252 to thin the thick wafer 242 to anappropriate thickness for the resulting device (e.g., 100 to 200 micronsfor a Schottky diode or MOSFET). The approach disclosed herein reducedthe need for wafer grinding, and instead utilizes laser- andcarrier-assisted separation to remove a portion of the thick wafer sothat it can be surface finished and used to fabricate another operativesemiconductor device.

The inventors have found that presence of the rounded edge 247 on thethick wafer 242 inhibits controlled formation of subsurface laser damageproximate to the edge 247, since the rounded profile negatively affectslaser focus and depth control. To address this issue, the rounded edge247 of the thick wafer 242 may be removed prior to further laserprocessing. FIG. 37J shows the structure of FIG. 37I being subjected togrinding with an edge grinder 256 to grind away the rounded edge 247 andimpart a substantially perpendicular edge 255 extending between thefirst and second surfaces 251, 252 of the thick wafer 242, with theepitaxial layers 253 and contacts 254 arranged over the first surface251.

FIG. 37K shows the structure of FIG. 37J following addition of temporaryadhesive material 257 over the first surface 251 of the thick wafer 242,the epitaxial layers 253, and the contacts 254, in preparation forreceiving and adhering a first carrier. FIG. 37L shows the structure ofFIG. 37K following addition of a first carrier 258 over the temporaryadhesive material 257, and following formation of subsurface laserdamage 259 within the thick wafer 242 by impingement of focused laseremissions through the second surface 252 of the thick wafer 242. FIG.37M shows the structure of FIG. 37L following bonding of a rigid secondcarrier 260 to the second surface 252 of the thick wafer 242 proximateto the subsurface laser damage 259. For purposes of separation, therigid second carrier 260 will serve as a frontside carrier intended toremove a portion (i.e., a layer) of the thick wafer 242.

In certain embodiments, laser emissions can be applied to a freestandingdevice thick wafer, and first and second carriers may be bonded to thefrontside and backside of the thick wafer at substantially the sametime. In certain embodiments, adhesive material may be applied oncarriers or the wafers for one or both of the front and back sides.

FIG. 37N shows the items of FIG. 37M following application of at leastone fracturing process as disclosed herein to fracture the thick wafer242 along the subsurface laser damage 259 to yield first and secondbonded subassemblies 262A, 262B. The first bonded subassembly 262Aincludes a first thin wafer portion 242A (separated from the thick wafer242 of FIG. 37M), the epitaxial layers 253, the contacts 254, thetemporary adhesive material, and the first carrier 258. The secondbonded subassembly 262B includes a second thin wafer portion 242B(separated from the thick wafer 242 of FIG. 37M) and the second carrier260. Exposed surfaces 259A, 259B of the thin wafer portions 242A, 242Bmay exhibit surface irregularities due to laser damage and/or fracturingthat may be reduced by conventional surface processing steps (e.g.,grinding, CMP, and/or polishing). FIG. 37O shows an operativesemiconductor device 264 derived from the first bonded subassembly 262Aby removal of the temporary adhesive 257 and the first carrier 258. Suchfigure also the second thin wafer portion 242B following removal of thesecond carrier 260, to prepare the second thin wafer portion 242B forfurther processing (e.g., epitaxial growth).

Exemplary Method Including Re-Use of Carrier Wafers

FIG. 38 is a flowchart schematically illustrating steps of a methodaccording to the present disclosure. Starting at upper left, a laser 266may focus laser emissions below a first surface 272 of a thickcrystalline material substrate 270 (e.g., a SiC ingot) to produce asubsurface laser damage region 268. Thereafter, a carrier wafer 224 maybe bonded to the first surface 272 of the crystalline material substrate270, with the carrier wafer 274 including a first surface 276 (proximalto the first surface 272 of the substrate 270) and a second surface 278that opposes the first surface 276 of the carrier wafer 274. Suchbonding between the carrier wafer 278 and the crystalline materialsubstrate 270 may be performed by any method disclosed herein, such asadhesive bonding or anodic bonding. Details concerning anodic bondingbetween crystalline material substrates and carriers are disclosed inU.S. Patent Application Publication No. 2016/0189954, with the contentsof such publication hereby being incorporated by reference herein, forall purposes. Thereafter, a fracturing process as disclosed herein(e.g., cooling a CTE mismatched carrier, application of ultrasonicenergy, and/or application of mechanical force) is applied to fracturethe crystalline material 270 along the subsurface laser damage region218, causing a crystalline material portion 280 bound to the carrierwafer 278 to be separated from a remainder of the crystalline materialsubstrate 270A. A newly exposed surface 282A of the remainder of thecrystalline material substrate 270A having residual laser damage isground smooth and cleaned, and returned to the beginning of the process(at upper left in FIG. 38). Also, a newly exposed surface 284 of theremoved crystalline material 280 is ground smooth while attached to thecarrier 274. Thereafter, the carrier wafer 274 may be separated from theremoved portion of the crystalline material 280, and the crystallinematerial 280 may be subject to epitaxial growth of one or more layers toform an epitaxial device 280′, while the carrier wafer 274 is cleanedand returned to the beginning of the process (at upper left in FIG. 38)to effectuate removal of another relatively thin section of thecrystalline material substrate 270.

FIG. 39 is a cross-sectional schematic view of a portion of thecrystalline material substrate (e.g., SiC ingot) 270 of FIG. 38 showingsubsurface laser damage 268 with superimposed dashed lines identifyingan anticipated kerf loss material region 290. The anticipated kerf lossmaterial region 290 includes laser damage 268, plus material 284 to bemechanically removed (e.g., by grinding and polishing) from a lower face288 (e.g., Si-terminated face) of the crystalline material portion 280(e.g., SiC wafer) to be separated from the substrate 270, plus material286 to be mechanically removed (e.g., by grinding and polishing) from anupper face 282A (e.g., C-terminated) face of the remainder 270A of thesubstrate 270. The lower face 288 of the crystalline material portion280 opposes an upper face 272 thereof. In certain embodiments, theentire kerf loss material region may have a thickness in a range of from80-120 microns for SiC to provide a substrate upper face 282A and awafer lower face 288 sufficient for further processing.

Material Processing with Multiple Grinding Stations/Steps

In certain embodiments, crystalline material subjected to laserprocessing and fracturing may be further processed with multiple surfacegrinding steps to remove subsurface damage and edge grinding to impart abeveled or rounded edge profile, wherein an order of grinding steps isselected and/or a protective surface coating is employed to reduce thelikelihood of imparting additional surface damage and to render acrystalline material wafer ready for chemical mechanical planarization.Such steps may be performed, for example, using material processingapparatuses according to embodiments disclosed herein, wherein anexemplary apparatus includes a laser processing station, a fracturingstation, multiple coarse grinding stations arranged in paralleldownstream of the fracturing station, and at least one fine grindingstation arranged downstream of the coarse grinding stations. Whenprocessing wafers cut by wire sawing, it is commonplace to perform edgegrinding prior to surface grinding or polishing to remove wire-sawingsurface damage. However, it has been found by the inventors that edgegrinding of substrate portions (e.g., wafers) having laser damage incombination with fracture damage, increases the likelihood of cracking asubstrate portion. While not wishing to be bound by any specific theoryas to the reason for this phenomenon, it is believed that exposed cleaveplanes resulting from surface fracturing renders the surfacessusceptible to cracking if edge grinding is performed prior to at leastsome surface processing (grinding and/or polishing). For this reason, ithas been found to be beneficial to perform at least some surfaceprocessing (e.g., grinding and/or polishing) prior to edge grinding.

It has been found that coarse grinding steps (i.e., to remove laserdamage and fracture damage along fractured surfaces of a substrateportion and a bulk substrate) tend to require significantly longer tocomplete than the preceding steps of laser processing and fracturing,and significantly longer than subsequent steps of fine grinding. Forthat reason, multiple coarse grinding stations are provided in parallelto remove a bottleneck in fabrication of multiple wafers from a bulkcrystalline material (e.g., an ingot). In certain embodiments, robotichandlers may be arranged upstream and downstream of the multiple coarsegrinding stations to control loading and unloading of substrateportions. In certain embodiments, a carrier bonding station may beprovided between a laser processing station and a fracturing station,and a carrier removal station may be provided upstream (either directlyor indirectly) of an edge grinding station. A carrier may desirablyremain bonded to a substrate portion during at least some surfacegrinding steps to reduce the potential for breakage, particularly forthin substrate portions (e.g., wafers); however, the carrier ispreferably removed prior to edge grinding (or prior to coating waferwith a protective coating preceding edge grinding).

In certain embodiments, a carrier bonding station may use carrierspre-coated with temporary bonding media, align and press the carrier toa substrate surface, and subject the bonding media with the necessaryconditions (e.g., heat and pressure) to effectuate bonding between thecarrier and the substrate. Alternatively, a carrier bonding station mayinclude a coating station that may be used to coat the carriers orsubstrates on demand.

FIG. 40 is a schematic illustration of a material processing apparatus300 according to one embodiment, including a laser processing station302, a carrier bonding station 303, a material fracturing station 304,multiple coarse grinding stations 308A, 308B arranged in parallel, afine grinding station 312, a carrier removal station 313, and a CMPstation 314. The laser processing station 302 includes at least onelaser, and a holder for at least one substrate arranged to receive atleast one laser beam for formation of subsurface laser damage in acrystalline material (e.g., an ingot). The carrier bonding station 303is configured to bond the crystalline material (having subsurface laserdamage therein) to at least one rigid carrier. The fracturing station304 is arranged to receive one or more assemblies (each including asubstrate bonded to a rigid carrier) from the carrier bonding station303, and to fracture the at least one substrate along a subsurface laserdamage region to remove a substrate portion (which may resemble a waferbonded to a carrier). First and second coarse grinding stations 308A,308B are arranged in parallel downstream of the fracturing station 304,with a first robotic handler 306 provided to alternately deliversubstrate portions (as part of bonded assemblies) received from thefracturing station 304 to either the first coarse grinding station 308Aor the second coarse grinding station 3048B. Downstream of the first andsecond coarse grinding stations 308A, 308B, a second robotic handler 310is provide to deliver coarse ground substrate portions (as part ofbonded assemblies) to a fine grinding station 312. A carrier removalstation 313 is provided downstream of the fine grinding station 312, andserves to separate ground substrate portions from carriers. A chemicalmechanical planarization (CMP) station 314 is arranged downstream of thecarrier removal station 313 to prepare substrate portions for furtherprocessing, such as cleaning and epitaxial growth. The CMP station 314functions to remove damage remaining after fine grinding, which itselfremoves damage remaining after coarse grinding. In certain embodiments,each coarse grinding station 308A, 308B comprises at least one grindingwheel having a grinding surface of less than 5000 grit, and the finegrinding station 312 comprises at least one grinding wheel having agrinding surface of at least 5000 grit. In certain embodiments, eachcoarse grinding station 308A, 308B is configured to remove a thicknessof 20 microns to 100 microns of crystalline material from a crystallinematerial portion (e.g., wafer), and the fine grinding station 312 isconfigured to remove a thickness of 3 to 15 microns of crystallinematerial. In certain embodiments, each coarse grinding station 308A,308B and/or fine grinding station 312 may include multiple grindingsubstations, in which different substations comprise grinding wheels ofdifferent grits.

An apparatus according to that of FIG. 40 may be modified to accommodateedge grinding to impart a rounded or beveled edge profile of acrystalline substrate portion, such as a wafer. Such an edge profilewill reduce the risk of breakage of a wafer edge. The edge grinding maynot be performed when a substrate portion is bonded to a carrier;accordingly, a carrier removal station may be arranged upstream (eitherdirectly or indirectly) of an edge grinding station.

FIG. 41 illustrates a material processing apparatus 320 according to oneembodiment similar to that of FIG. 40, but incorporating an edgegrinding station 332. The material processing apparatus 320 includes alaser processing station 322, a carrier bonding station 323, a materialfracturing station 324, a first robotic handler 326, multiple coarsegrinding stations 328A, 328B arranged in parallel, a second robotichandler 328, a carrier removal station 331, an edge grinding station332, a fine grinding station 334, and a CMP station 336. An exemplaryedge grinding station 332 may be arranged to grip a wafer between upperand lower gripping portions of a turntable arranged proximate to arotary grinding tool having a concave during surface (e.g., such asillustrated in FIG. 37G). Gripping of a wafer in this manner mayundesirably impart damage to a wafer surface (e.g., a Si-terminatedsurface of a SiC wafer). For this reason, the edge grinding station 332shown in FIG. 41 is arranged upstream of the fine grinding station 334,to permit any surface damage imparted by the edge grinding station 332to be removed in the fine grinding station 334. Although the finegrinding station 334 may remove a small degree of thickness of a wafer,thereby altering a rounded or beveled edge profile produced by the edgegrinding station 332, a sufficient degree of a rounded or beveled edgeprofile will remain to inhibit fracture of a wafer edge.

The apparatus 320 according to FIG. 41 may be used to perform a methodfor processing a crystalline material wafer comprising a first surfacehaving surface damage thereon, with the first surface being bounded byan edge. The method comprises grinding the first surface with at leastone first grinding apparatus to remove a first part of the surfacedamage; following the grinding of the first surface with the at leastone first grinding apparatus, edge grinding the edge to form a beveledor rounded edge profile; and following the edge grinding, grinding thefirst surface with at least one second grinding apparatus to remove asecond part of the surface damage sufficient to render the first surfacesuitable for further processing by chemical mechanical planarization. Incertain embodiments, the first grinding apparatus may be embodied in thecoarse grinding stations 328A, 328B, the edge grinding may be performedby the edge grinding station 332, and the second grinding apparatus maybe embodied in the find grinding station 312. In certain embodiments, acarrier removal step may be performed following the grinding of thefirst surface with the at least one first grinding apparatus, and priorto edge grinding the edge to form the beveled or rounded edge profile.

In certain embodiments, a protective surface coating may be employed toreduce the likelihood of imparting additional surface damage during edgegrinding and to render a crystalline material wafer ready for chemicalmechanical planarization. Such a surface coating may include photoresistor any other suitable coating material, may be applied prior to edgegrinding, and may be removed after edge grinding.

FIG. 42 is a schematic illustration of a material processing apparatus340 according to one embodiment similar to that of FIG. 40, butincorporating a surface coating station 354 between a fine grindingstation 352 and an edge grinding station 356, and incorporating acoating removal station 358 between the edge grinding station 356 and aCMP station 360. The material processing apparatus 340 further includesa laser processing station 342, a material fracturing station 344, afirst robotic handler 346, multiple coarse grinding stations 348A, 348Barranged in parallel, and a second robotic handler 348 upstream of thefine grinding station 352. The coating station 354 may be configured toapply a protective coating (e.g., photoresist) by a method such as spincoating, dip coating, spray coating, or the like. The protective coatingshould be of sufficient thickness and robustness to absorb any damagethat may be imparted by the edge grinding station 365. For a SiC wafer,the Si-terminated surface may be coated with the protective coating,since the Si-terminated surface is typically the surface on whichepitaxial growth is performed. The coating removal station 358 may beconfigured to strip the coating by chemical, thermal, and/or mechanicalmeans.

The apparatus 340 according to FIG. 42 may be used to perform a methodfor processing a crystalline material wafer comprising a first surfacehaving surface damage thereon, with the first surface being bounded byan edge. The method comprises grinding the first surface with at leastone first grinding apparatus (e.g., the coarse grinding stations 348A,348B) to remove a first part of the surface damage; thereafter grindingthe first surface with at least one second grinding apparatus (e.g., thefine grinding station 352) to remove a second part of the surface damagesufficient to render the first surface suitable for further processingby chemical mechanical planarization; thereafter forming a protectivecoating on the first surface (e.g., using the surface coating station354); thereafter edge grinding the edge to form a beveled or roundededge profile (e.g., using the edge grinding station 356); and thereafterremoving the protective coating from the first surface (e.g., using thecoating removal station). The first surface may thereafter be processedby chemical mechanical planarization (e.g., by the CMP station 360),thereby rendering the first surface (e.g., a Si terminated surface ofthe wafer) ready for subsequent processing, such as surface cleaning andepitaxial growth.

In certain embodiments, a gripping apparatus may be configured forholding an ingot having end faces that are non-perpendicular to asidewall thereof to permit an end face to be processed with a laser forformation of subsurface damage. In certain embodiments, grippingeffectors may conform to a sloped sidewall having a round cross-sectionwhen viewed from above. In certain embodiments, gripping effectors mayinclude joints to permit gripping effectors to conform to the slopedsidewall.

FIG. 43A is a schematic side cross-sectional view of a first grippingapparatus 362 for holding an ingot 364 having end faces 366, 368 thatare non-perpendicular to a sidewall 370 thereof, according to oneembodiment. The upper end face 366 is horizontally arranged to receive alaser beam 376. The lower end face 368 may have a carrier 372 attachedthereto, with a chuck 374 (e.g., a vacuum chuck) retaining the carrier372. Gripping effectors 378 having non-vertical faces are provided togrip sidewalls 370 of the ingot 364, wherein the gripping effectors 378are arranged at non-perpendicular angles A1, A2 relative to horizontalactuating rods 380. Holding the ingot 364 as shown (e.g., proximate to abottom portion thereof) using the gripping apparatus 362 leaves theupper end face 366 and upper portions of the sidewall 370 available forprocessing using methods disclosed herein.

FIG. 43B is a schematic side cross-sectional view of a second grippingapparatus 362′ for holding an ingot 364′ having end faces 366′, 368′that are non-perpendicular to a sidewall 370′ thereof, according to oneembodiment. The upper end face 366′ is horizontally arranged to receivea laser beam 376, whereas the lower end face 368′ may have a carrier372′ attached thereto, with the carrier 372′ retained by a chuck 374′.Gripping effectors 378′ having non-vertical faces are provided to gripsidewalls 370′ of the ingot 364′, wherein the gripping effectors 378′are arranged at non-perpendicular angles A1, A2 relative to horizontalactuating rods 380′. Pivotable joints 382′ are provided between theactuating rods 380′ and the gripping effectors 378′, therebyfacilitating automatic alignment between the gripping effectors 378′ andsidewalls 370′ of the ingot 364′.

In one example, a 150 mm diameter single crystal SiC substrate (ingot)having a thickness of more than 10 mm is used as a starting material forproduction of a SiC wafer having a thickness of 355 microns. Laseremissions are impinged through a C-terminated upper face of the SiCsubstrate to form subsurface laser damage. A sapphire carrier is bondedto the upper face of the SiC substrate using a thermoplastic adhesivematerial disclosed herein, and thermal-induced fracture is performed toseparate an upper (wafer) portion of SiC from a remainder of the ingot.Both the Si-terminated face of the separated wafer portion and theC-terminated face of the ingot remainder are coarse ground using a 2000grit grind wheel (e.g., a metal, vitreous, or resin bond-type grindingwheel) to removal all visible laser and fracture damage. Thereafter,both the Si-terminated face of the separated wafer portion and theC-terminated face of the ingot remainder are fine ground (e.g., using avitreous grinding surface) with a 7000 or higher grit (e.g., up to30,000 grit or higher) to yield smoother surfaces, preferably less than4 nm average roughness (R_(a)), more preferably in a range of 1-2 nmR_(a). On the ingot remainder, a smooth surface is required to avoid anyimpact on the subsequent laser processing. The wafer is to be CMP readyand of sufficient smoothness to minimize required CMP removal amounts,since CMP is typically a higher cost process. Typical material removalduring fine grind processing may be in a thickness range of 5 to 10microns to remove all residual subsurface damage from the coarse grindand any remaining laser damage (both visible and non-visible to thenaked eye). Thereafter, the ingot remainder is returned to a laser forfurther processing, and the wafer is edge ground and subjected tochemical mechanical planarization (CMP) to be ready for epitaxialgrowth. Edge grinding may be performed between coarse and fine surfacegrinding to avoid any risk of scratching the fine ground Si face.Material removal during CMP may be in a thickness range of about 2microns. Total material consumed from the substrate (ingot) may be lessthan 475 microns. Given the 355 micron final wafer thickness, the kerfloss is less than 120 microns.

Variability in Wafer-to-Wafer Thickness Influenced by Laser Power andCrystal Variation

As noted previously herein, progressively higher laser power levels maybe necessary for formation of laser damage sufficient to partcrystalline material by fracturing, starting at a position distal fromthe seed crystal and obtaining wafers at cross-sectional positionsprogressively approaching the seed crystal. Use of high laser power ateach sequential depth position when forming subsurface damage wouldentail unnecessary material loss, and would also significantly increasewafer-to-wafer thickness spread due to variability in both the damagedepth and the point at which decomposition is reached relative to alaser beam waist. Such concept may be understood with reference to FIGS.44 and 45.

FIG. 44 is a schematic side cross-sectional view of a conventional laserfocusing apparatus that focuses an incoming horizontal beam 400 in apropagation direction with a lens 404, forming an outgoing beam 402having a beam waist pattern having a minimum width W_(f) at a position406 corresponding to a focal length f of the lens 404. Downstream ofthis position 406, the beam width broadens to a wider region 408. FIG.45 is a schematic side cross-sectional view of a vertically orientedfocused laser beam 402 that may be directed into a crystalline materialand exhibits a beam waist pattern (with a minimum width at a position406 corresponding to a focal length of a lens (not shown)), with thebeam width broadening thereafter to a wider region 408. When the focusedlaser beam 402 is directed within a crystalline material (e.g., asubstrate such as a SiC ingot), the crystalline material will thermallydecompose at different threshold points (i.e., depths) depending onfactors such as laser power, degree of absorption of radiation by thecrystalline material (which may be influenced by presence or absence ofdopants and/or crystal defects that may vary with depth (and width)position within the substrate), and degree of focusing which isdependent on vertical position. Three different decomposition thresholdpoints 410A-410C are shown in FIG. 45.

Methods and apparatuses disclosed herein permit the foregoing issues tobe addressed by imaging a top surface of a crystalline materialsubstrate having subsurface laser damage to detect uncracked regionswithin the substrate, analyzing one or more images to identify acondition indicative of presence of uncracked regions within thesubstrate, and taking one or more actions responsive to the analyzing(e.g., upon attainment of appropriate conditions). Such actions mayinclude performing an additional laser pass at the same depth positionand/or changing an instruction set for producing subsurface laser damageat subsequent depth positions). Such methods and apparatuses facilitateproduction of substrate portions of uniform thickness withoutunnecessary material loss.

FIGS. 46A-46C provide plots of laser power versus sequential waferidentification for wafers (i.e., sequential wafer identification (ID)numbers from 1 to 55) derived from three SiC ingots, respectively,wherein proximity to the seed crystal increases with increasing wafer IDnumber in each instance (i.e., with slice 1 being farthest from the seedcrystal). FIG. 46A shows results for a first SiC ingot, in which a firstwafer group 411A was fractured after formation of subsurface laserdamage at a laser power level of about 3.75 W, a second wafer group 412Awas fractured after formation of subsurface laser damage at a laserpower level of about 4 W, a third wafer group 413A was fractured afterformation of subsurface laser damage at a laser power level of about4.25 W, a fourth wafer group 414A was fractured after formation ofsubsurface laser damage at a laser power level of about 4.5 W, and afifth wafer group 415A was fractured after formation of subsurface laserdamage at a laser power level of about 4.6 W. FIG. 46B shows results fora second SiC ingot, in which a first wafer group 411B was fracturedafter formation of subsurface laser damage at a laser power level below3.2 W, a second wafer group 412B was fractured after formation ofsubsurface laser damage at a laser power level of about 3.4 W, and thirdthrough fifth wafer groups 413B-415B were fractured after formation ofsubsurface laser damage at higher laser power levels each about 0.25 Wgreater than the last. FIG. 46C shows that ten different laser powerlevels were required to successfully form fifty-five wafers sequentiallyparted (by formation of subsurface laser damage followed by fracturing)from a single SiC ingot, according to wafer groups 411C-420C at laserpower levels ranging from about 4 W to about 5.5 W among the differentwafer groups 411C-420C. FIGS. 46A-46C therefore show significantvariation in laser power requirements from ingot to ingot, as well aswithin each ingot, to form multiple wafers of substantially the samethickness by laser-assisted parting.

FIG. 47 is a plot of resistivity (Ohm-cm) versus slice number for fiftywafers per ingot produced from about fifty (50) SiC ingots, with asuperimposed polynomial fit showing resistivity decreasing with slicenumber. In each instance, an increasing slice number representsincreasing proximity to a seed crystal on which the ingot wasepitaxially grown, with slice 1 representing the top of the ingotfarthest from the seed crystal. Although resistivity ranges varysignificantly from ingot to ingot, resistivity consistently decreasesthroughout each ingot with increasing proximity to the seed crystal. Theresistivity value range of the y-axis of FIG. 47 is consistent withN-type SiC. A reduction in resistivity corresponds to an increase indoping and an increase in laser absorption.

FIG. 48 is a plot of laser power (Watts) versus resistivity for wafersproduced from about fifty (50) SiC ingots, with a superimposedpolynomial fit, with the laser power representing a value need toachieve successful laser-assisted parting (with fracturing followingsubsurface laser damage formation) by a method described herein. FIG. 48shows that, although laser power requirements vary significantly fromingot to ingot, laser power levels necessary to achieve successfulparting decrease with increasing resistivity levels of the ingot.

Apparatus Including Diffuse Light Source and Imaging Device

In certain embodiments, a material processing apparatus includes a laserprocessing station configured to process a substrate of crystallinematerial to form subsurface laser damage therein, with the laserprocessing station including illumination and imaging devices configuredto permit detection of conditions indicative of presence of uncrackedregions in the interior of the crystalline material. Using uncrackedregions as a visible indicator to determine when additional lasersubstrate damage is necessary at a first average depth position (forformation of a first reduced thickness portion of a substrate, such as afirst wafer derived from an ingot) and/or when additional laser power isnecessary for formation of laser damage at subsequent average depthpositions (for formation of a subsequent reduced thickness portions of asubstrate, such as subsequent wafers derived from an ingot), a stableand repeatable laser parting process can be provided in terms of waferthickness distribution while avoiding unnecessary kerf losses. The term“average depth position” is used in this context since slight variationin laser focus depth positions (e.g., typically 10 microns or less) maybe used between subsurface laser damage formation passes for forming thesame reduced thickness portion of the substrate and/or within a singlelaser damage formation pass (e.g., to address the presence of anincreased doping region such as a doping ring).

Preferably, the illumination and imaging devices are positioned topermit imaging of a substrate surface while the substrate is retained bya laser processing chuck. Such capability permits a substrate to beinspected (e.g., imaged and analyzed in an automated manner) to rapidlyassess whether additional laser processing may be necessary prior tofracturing, without requiring the substrate to be removed andreinstalled in a laser processing chuck. This in situ inspection of asubstrate while present in a laser processing station increases laserprocessing tool utilization by avoiding downtime, thereby enhancinglaser parting process throughput. In certain embodiments, a laser may bemoved away from a substrate retained by a laser processing chuck topermit imaging to be performed without the laser blocking illuminationor imaging of the substrate surface.

FIGS. 49A and 49B provide schematic side cross-sectional and top planviews, respectively, of a diffuse light source 438 and an imaging device442 arranged proximate to a crystalline material substrate 430 within alaser processing station 425. Referring to FIG. 49A, the crystallinematerial substrate 430 includes a top surface 433 and subsurface laserdamage 434 arranged within an interior of the substrate 430 below thetop surface 433. The subsurface laser damage 434 may roughly resemble anirregular sawtooth pattern in a direction parallel to the <11-20>direction if the substrate has a hexagonal crystal structure. Thesubstrate 430 includes a central axis 436. The diffuse light source 438is laterally displaced in a first direction 437A relative to the centralaxis 436, and the imaging device 442 is laterally displaced in anopposing, second direction 437B relative to the central axis 436. Boththe diffuse light source 438 and the imaging device 442 may be upwardlydisplaced relative to the top surface 433 of the substrate 430.Additionally, the diffuse light source 438 may be arranged to a firstlateral side 431 of the substrate 430, and the imaging device 442 may bearranged to an opposing, second lateral side 432 of the substrate 430.In certain embodiment, an angle definable between a light emittingsurface of the diffuse light source 438 and a light receiving surface ofthe imaging device 442 (optionally representable as an angle betweenbeams 440 exiting the diffuse light source 438 and an incident lightbeam 444 received by the imaging device 442) may be in a range betweenabout 100 degrees and about 170 degrees. In certain embodiments, thediffuse light source 438 may include any suitable one or more lightemitting devices (e.g., light emitting diodes), with a diffuser arrangedbetween the light emitting device(s) and light beams 440 exiting thediffuse light source 438. In certain embodiments, the imaging device 442may include one or more charge-coupled device (CCD) or complementarymetal-oxide semiconductor (CMOS) image sensors, optionally arranged inan array.

FIG. 49B provides a top plan view of the same elements as depicted inFIG. 49A. The substrate 430 may include a primary flat 435 (shown inFIG. 49B) that is substantially parallel to the <11-20> direction (shownin FIG. 49A). The inventors have found orientation of the diffuse lightsource 438 and the imaging device 442 relative to the substrate 430 tobe important to aid in imaging of subsurface laser damage. In certainembodiments, the light source may be positioned substantiallyperpendicular to the primary flat 435 of the substrate 430 and/or within±5 degrees of perpendicular to a <1120> direction of a hexagonal crystalstructure of the substrate 430, to enhance visibility of uncrackedregions through the top surface 433 of the substrate 430.

FIG. 50A is an image of a top surface of a crystalline SiC substrate 450having subsurface laser damage and imaged with an apparatus similar tothat shown in FIGS. 49A-49B. The substrate 450 of FIG. 50A includesthree regions 451-453 of different colors (in the original image) andirregularly shaped dark regions 456 corresponding to uncracked regionswithin the substrate 450. In the original image, an outermostapproximately annular region 451 (including a primary flat 455) ispredominantly green in color, an intermediate approximately annularregion 452 is predominantly red in color, and a central approximatelycircular region 453 is predominantly gold in color. Within the centralregion 453, irregularly-shaped black regions 456 are visible. The blackregions 456 correspond to presence of uncracked regions along subsurfacelaser damage below an upper surface of the substrate 450. The differentcolors (green, red, and gold) of the different regions 451-453 arebelieved to correspond to degree of cracking damage. In certainembodiments, a method disclosed herein comprises analyzing at least oneproperty of the image to identify areas having different degrees ofcracking damage, and adjusting one or more laser damage formationparameters (e.g., laser power, laser focusing depth, laser pulseduration, and/or number of laser damage formation passes, or the like)for formation of supplemental subsurface laser damage at the same (e.g.,first) average depth position (to form a first reduced thickness portionof a substrate, such as a first wafer) and/or formation of subsequentsubsurface laser damage at different (e.g., second or subsequent)average depth positions (to form second and subsequent reduced thicknessportions of the substrate, such as second and subsequent wafers) toaddress differences in substrate properties that may vary with vertical(depth) position and/or horizontal position within the substrate.

Although FIGS. 49A-49B illustrate use of a diffuse light source 438 andan imaging device 442 arranged along opposing lateral sides of asubstrate, in certain embodiments other configurations and/or types oflight sources and imaging devices may be used. In certain embodiments,one or more portions (or an entirety) of a top surface of a substratemay be scanned with at least one microscope, such an optical microscope,confocal microscope, a scanning electron microscope, and/or atransmission electronic microscope, with relative translation betweenthe substrate and the microscope(s).

FIG. 50B is a schematic diagram of a substrate representation 450Ashowing the irregularly shaped dark regions 456 of FIG. 50A withindotted line regions 451A-453A (of which the outermost region 453Aencompasses a primary flat 455A) substantially corresponding toboundaries between differently-colored regions 451-453 of the topsurface of the substrate 450 of FIG. 50A. In certain embodiments, allregions except the dark regions 456 may be removed from a captured imageto facilitate analysis of one or more area properties of the darkregions 456.

FIG. 50C is a magnified view of the irregularly shaped dark regions 456shown in FIGS. 50A-50B, but with continuous dark regions beingindividually numbered 456A-456D and with addition of rectangular boxesaround individual regions 456A-456D. Each dark region 456A-456D has amaximum length and maximum width, corresponding to L₁-L₄ and W₁-W₄,respectively. In certain embodiments, analysis of an image derived fromthe substrate includes identification of a condition indicative ofpresence of uncracked regions in the interior of the crystallinematerial (e.g., dark and/or black regions in certain embodiments), andquantification of a top area property (or at least one top areaproperty) of one or more uncracked regions in the interior. In certainembodiments, a quantified top area property includes an aggregate toparea of all uncracked regions. In certain embodiments, a quantified toparea property includes separate identification of any continuousuncracked regions, together with quantification of top area of eachcontinuous uncracked region and/or quantification of maximum length andwidth dimensions of the continuous uncracked regions and/oridentification of length/width aspect ratio of the continuous uncrackedregions. In certain embodiments, the length and width may be establishedrelative to crystallographic direction and/or a primary flat of thesubstrate (e.g., with length perpendicular to the primary flat, and withwidth parallel to the primary flat). The inventors have found thatpresence of large continuous uncracked regions of a given top area mayinhibit fracture more readily than presence of numerous discontinuousuncracked regions of the same aggregate top area. Additionally, theinventors have found that orientation and/or aspect ratio of continuousuncracked regions may affect fracture inhibition. Small localized blackregions indicative of uncracked regions generally do not impedeseparation by fracturing, but as the black regions increase in size(particularly in a width direction generally parallel to the primaryflat and/or generally perpendicular to laser damage lines), such regionsmay identify the need for adding another laser damage formation pass atthe same average depth position and/or to increase laser power whenforming laser damage regions at subsequent average depth positions.Uncracked regions having a large length (e.g., in a directionperpendicular to the primary flat) may be less problematic in inhibitingfracture than uncracked regions having a large width.

FIG. 51 is a schematic illustration of a material processing apparatus458 according to one embodiment, including a laser processing station459 that includes a laser 465, at least one translation stage 466 (e.g.,preferably x-y-z translation stages) configured to promote relativemovement between the laser 465 and a substrate 460), a diffuse lightsource 468 configured to illuminate a top surface 463 of the substrate460, and an imaging device 472 configured to generate at least one imageof the top surface 463 of the substrate 460. The substrate 460 isarranged above a support 464, which may include a laser tool chuck.Various items within the material processing apparatus 458 are inelectrical communication with at least one computing device 474 havingan associated memory 476. The computing device 474 may control operationof the diffuse light source 468, the imaging device 472, the laser 465,and the translation stage(s) 466. The memory 476 may further storesubstrate-specific instruction sets (e.g., fabrication recipes) that maybe used and modified on an individual substrate basis. In certainembodiments, the computing device 474 and memory 476 may be used inperforming various steps of methods disclosed herein, including but notlimited to analyzing substrate images to identify conditions indicativeof presence of uncracked regions in the interior of substrates,quantification of one or more top area properties of uncracked regions,and comparing top area properties to one or more predetermined thresholdarea properties. In certain embodiments, the computing device 474 mayfurther be used to analyze images and detect presence of differentlycolored regions (e.g., other than presence of black or dark spots) andadjust operation of the laser 465 to compensate for laser facetcontamination responsive to such analysis. In certain embodiments, thecomputing device 474 may further be used to detect presence of differentdoping conditions in different regions of a substrate, and responsivelyalter laser power delivery. In certain embodiments, responsive todetection of the condition indicative of non-uniform doping of thecrystalline material, laser power may be altered during formation ofsubsurface laser damage patterns to provide laser emissions at a firstaverage power level when forming subsurface laser damage in a firstdoping region, and to provide laser emissions at a second average powerlevel when forming subsurface laser damage in a second doping region,wherein the first and second average power levels differ from oneanother.

Methods Including Imaging, Comparison, and Lasering/Power Adjustment

FIG. 52 is a flowchart 480 illustrating steps in a first crystallinematerial processing method that generally includes generating an imageof a top surface of a substrate having subsurface laser damage,analyzing the image to identify presence of a condition indicative ofone or more uncracked regions, comparing one or more properties of theuncracked regions to first and second thresholds, and taking action(i.e., (A) performing an additional laser pass at substantially the samedepth position to form supplemental laser damage, optionally withadjustment of one or more laser parameters and/or (B) adjusting one ormore laser parameters for forming subsurface laser damage at second andsubsequent depth positions) responsive to the comparisons. The method isinitiated at block 482. Proceeding to block 484, a first step includesforming subsurface laser damage in a crystalline material substratehaving at least one subsurface laser damage pattern (optionallyincluding at least one plurality of substantially parallel lines asdisclosed herein) along a first (or new) depth position below a topsurface of the substrate, wherein the at least one subsurface laserdamage pattern is configured to promote formation of at least oneplurality of cracks in the interior of the crystalline materialpropagating outward from the at least one subsurface laser damagepattern. Proceeding to block 486, a second step includes generating atleast one image of the top surface of the substrate. In certainembodiments, the image generating step includes illuminating the topsurface with a diffuse light source arranged to a first lateral side ofthe substrate (preferably arranged substantially perpendicular to aprimary flat of the substrate and/or within ±5 degrees of perpendicularto a <1120> direction of the hexagonal crystal structure), and capturingthe at least one image with an imaging device arranged to a secondlateral side of the substrate that opposes the first lateral side. Incertain embodiments, one or more alternative or additional imagingmethods as disclosed herein may be used.

Proceeding to block 488, a further step includes analyzing the at leastone image to identify a condition indicative of presence of uncrackedregions in the interior of the crystalline material (e.g., dark and/orblack regions in certain embodiments). Optionally, at least one top areaproperty of one or more uncracked regions in the interior may bequantified, wherein a quantified top area property may optionallyinclude an aggregate top area of all uncracked regions. In certainembodiments, a quantified top area property includes separateidentification of any continuous uncracked regions, together withquantification of the top area of each continuous uncracked regionand/or quantification of maximum length and width dimensions of thecontinuous uncracked regions and/or identification of length/widthaspect ratio of the continuous uncracked regions. In certainembodiments, the length and width may be established relative tocrystallographic direction and/or a primary flat of the substrate (e.g.,with length perpendicular to the primary flat, and with width parallelto the primary flat). The inventors have found that presence of largecontinuous uncracked regions of a given top area may inhibit fracturemore readily than presence of numerous discontinuous uncracked regionsof the same aggregate top area. Additionally, the inventors have foundthat orientation and/or aspect ratio of continuous uncracked regions mayaffect fracture inhibition. Small localized black regions indicative ofuncracked regions generally do not impede separation by fracturing, butas the black regions increase in size (particularly in a width directiongenerally parallel to the primary flat and/or generally perpendicular tolaser damage lines), such regions may identify the need for addinganother laser damage formation pass at the same average depth positionand/or to increase laser power when forming laser damage regions atsubsequent average depth positions. Uncracked regions having a largelength (e.g., in a direction perpendicular to the primary flat) may beless problematic in inhibiting fracture than uncracked regions having alarge width.

Proceeding to decision block 490, one or more properties of theuncracked region(s) (optionally including at least one quantified toparea property) is compared to at least one first predeterminedthreshold. The first threshold may include (without necessarily beinglimited to) any one or more of: a continuous uncracked region top areathreshold, an aggregate uncracked top area threshold, a maximumuncracked width threshold, a maximum length/width aspect ratiothreshold, or the like. If the at least one property of the uncrackedregion(s) does not exceed the at least one first predeterminedthreshold, then the method proceeds to block 498, according to which thesubstrate is transferred to a fracturing station for producing a firstreduced thickness portion of the substrate (e.g., a first wafer from aningot) generally corresponding in thickness to the first average depthposition. Conversely, if the at least one property of the uncrackedregion(s) does exceed the at least one first predetermined threshold,then the method proceeds to block 492, according to which an instructionset (e.g., fabrication recipe), associated with the substrate, ismodified by incrementally adjusting at least one laser parameter forformation of subsurface laser damage when producing subsurface laserdamage patterns at a second average depth position and any subsequentaverage depth positions in the substrate (e.g., for formation of atleast one additional reduced thickness portion of the substrate, such assecond and subsequent wafers from an ingot). Laser parameters that maybe adjusted include any one or more of laser power, laser focus depth,number of laser passes, laser pass spacing, laser pulse width, etc. Incertain embodiments involving alteration of laser power, the instructionset is modified to increase average laser power by a value in a range offrom 0.10 to 0.50 watts, or from 0.15 to 0.35 watts, or from 0.20 to0.30 watts, or by a value of about 0.25 watts. The adjustment of one ormore laser parameters for formation of subsequent laser damage at secondand subsequent average depth positions is not necessarily followed byformation of additional damage at the previously established firstaverage depth position. Determining whether additional laser damage atthe first average depth position may be necessary to promote fracture isperformed at decision block 494.

Decision block 494 includes a step of comparing one or more propertiesof uncracked region(s) (optionally including at least one quantified toparea property) to at least one second predetermined threshold. Incertain embodiments, the second predetermined threshold is greater thanthe first predetermined threshold. The second threshold may include(without necessarily being limited to) any one or more of: a continuousuncracked region top area threshold, an aggregate uncracked top areathreshold, a maximum uncracked width threshold, a maximum length/widthaspect ratio threshold, or the like. If the at least one property of theuncracked region(s) does not exceed the at least one secondpredetermined threshold, then the method proceeds to block 498,according to which the substrate is transferred to a fracturing station,since no additional laser damage is deemed necessary to support fractureof the substrate along the first average depth position. Conversely, ifthe at least one property of the uncracked region(s) does exceed the atleast one second predetermined threshold, then the method proceeds toblock 496, according to which additional subsurface laser damage isformed along the first average depth position. In certain embodiments,this entails effecting relative movement between the laser and thesubstrate while supplying emissions of the laser focused within theinterior of the substrate, at least in the uncracked region(s) butoptionally over the entire substrate, to form supplemental subsurfacelaser damage at or proximate to the first average depth position tosupplement the at least one subsurface laser damage pattern and promoteformation of additional cracks in the interior of the crystallinematerial propagating outward from supplemented at least one subsurfacelaser damage pattern. Following formation of this supplementalsubsurface laser damage, the method proceeds to block 498, according towhich the substrate is transferred to a fracturing station.

Proceeding to block 500, in certain embodiments a carrier may be bondedto the substrate at the fracturing station to form a bonded assembly.Thereafter, according to block 502, the crystalline material isfractured along the first depth position to separate the bonded assembly(including the carrier and a removed portion of the substrate) from theremainder of the substrate, with such step serving to expose a new topsurface of the substrate. Thereafter, according to block 504, thesubstrate may be returned to the laser processing station (optionallyafter surface treatment such as grinding and/or polishing of the newlyexposed substrate surface) to enable performance of another subsurfacelaser damage step according to block 484. If the instruction setassociated with the substrate was modified to increase average laserpower according to block 492, then the modified instruction set will beused for formation of subsurface damage in performance of the stepdescribed at block 484. This modified instruction set is preferablystored in a memory and associated with the particular substrate (e.g.,in a record of a relational database including a substrate identifierand parameters for formation of subsurface laser damage within thesubstrate). In this manner, a substrate-specific recipe for formingsubsurface laser damage is maintained and may be dynamically updated.

Following fracturing of the bonded assembly from the substrate at block502, the bonded assembly may be transferred to one or more surfaceprocessing stations (according to block 506) to alter the substrateportion attached to the carrier. Examples of surface processing stepsthat may be performed include coarse grinding, edge grinding, finegrinding, and cleaning according to blocks 508, 510, 512, and 514,respectively. Thereafter, the processed substrate portion may be readyfor epitaxial growth.

FIG. 53 is a flowchart 520 illustrating steps in a second crystallinematerial processing method that generally includes generating an imageof a top surface of a substrate having subsurface laser damage,analyzing the image to quantify a top area property of one or moreuncracked regions, comparing the top area property to first and secondthreshold area properties, and taking action (i.e., perform anadditional laser pass at the same depth position and/or adjust power forsubsurface laser damage at subsequent depth positions) responsive to thecomparisons to enhance reliability of producing substrate portions(e.g., wafers) from the substrate. The method is initiated at block 522.Proceeding to block 524, a first step includes forming subsurface laserdamage in a crystalline material substrate having at least onesubsurface laser damage pattern (optionally including at least oneplurality of substantially parallel lines) along a (new) first depthposition below a top surface of the substrate, wherein the at least onesubsurface laser damage pattern is configured to promote formation of atleast one plurality of cracks in the interior of the crystallinematerial propagating outward from substantially the at least onesubsurface laser damage pattern. Proceeding to block 526, a second stepincludes generating at least one image of the top surface of thesubstrate. In certain embodiments, the image generating step includesilluminating the top surface with a diffuse light source arranged to afirst lateral side of the substrate (preferably arranged substantiallyperpendicular to a primary flat of the substrate and/or within ±5degrees of perpendicular to a <1120> direction of the hexagonal crystalstructure), and capturing the at least one image with an imaging devicearranged to a second lateral side of the substrate that opposes thefirst lateral side. In certain embodiments, one or more alternative oradditional imaging methods as disclosed herein may be used.

Proceeding to block 528, a further step includes analyzing the at leastone image to identify a condition indicative of presence of uncrackedregions in the interior of the crystalline material (e.g., dark and/orblack regions in certain embodiments), and to quantify a top areaproperty (or at least one top area property) of one or more uncrackedregions in the interior. In certain embodiments, a quantified top areaproperty includes an aggregate top area of all uncracked regions. Incertain embodiments, a quantified top area property includes separateidentification of any continuous uncracked regions, together withquantification of top area of each continuous uncracked region and/orquantification of maximum length and width dimensions of the continuousuncracked regions and/or identification of length/width aspect ratio ofthe continuous uncracked regions. In certain embodiments, the length andwidth may be established relative to crystallographic direction and/or aprimary flat of the substrate (e.g., with length perpendicular to theprimary flat, and with width parallel to the primary flat).

Proceeding to decision block 530, the at least one quantified top areaproperty is compared to at least one first predetermined area (or areaproperty) threshold. The first threshold may include any one or more of:a continuous uncracked region top area threshold, an aggregate uncrackedtop area threshold, a maximum uncracked width threshold, a maximumlength/width aspect ratio threshold, or the like. If the at least onequantified top area property does not exceed the at least one firstpredetermined threshold area property, then the method proceeds to block538, according to which the substrate is transferred to a fracturingstation. Conversely, if the at least one quantified top area propertydoes exceed the at least one first predetermined threshold areaproperty, then the method proceeds to block 532, according to which aninstruction set (e.g., fabrication recipe), associated with thesubstrate, is modified by incrementally increasing average laser powerfor formation of subsurface laser damage when producing subsurface laserdamage patterns at a second average depth position and any subsequentaverage depth positions in the substrate. (Laser parameters that may beadjusted may additionally or alternatively include any one or more oflaser focus depth, number of laser passes, laser pass spacing, laserpulse width, etc.) In certain embodiments, the instruction set ismodified to increase average laser power by a value in a range of from0.10 to 0.50 watts, or from 0.15 to 0.35 watts, or from 0.20 to 0.30watts, or by a value of about 0.25 watts. The incrementing of laserpower for formation of subsequent laser damage at second and subsequentaverage depth positions is not necessarily followed by formation ofadditional damage at the previously established first average depthposition. Determining whether additional laser damage may be necessaryto promote fracture is performed at decision block 534.

Decision block 534 includes a step of comparing the at least onequantified top area property to at least one second predeterminedthreshold area property. In certain embodiments, the secondpredetermined threshold area property is greater than the firstpredetermined threshold area property. The second threshold areaproperty may include any one or more of: a continuous uncracked regiontop area threshold, an aggregate uncracked top area threshold, a maximumuncracked width threshold, a maximum length/width aspect ratiothreshold, or the like. If the at least one quantified top area propertydoes not exceed the at least one second predetermined threshold areaproperty, then the method proceeds to block 538, according to which thesubstrate is transferred to a fracturing station, since no additionallaser damage is deemed necessary to support fracture of the substratealong the first average depth position. Conversely, if the at least onequantified top area property does exceed the at least one secondpredetermined threshold area property, then the method proceeds to block536, according to which supplemental subsurface laser damage is formedalong the first average depth position. In certain embodiments, thisentails effecting relative movement between the laser and the substratewhile supplying emissions of the laser focused within the interior ofthe substrate to form supplemental subsurface laser damage at orproximate to the first average depth position to supplement the at leastone subsurface laser damage pattern and promote formation of additionalcracks in the interior of the crystalline material propagating outwardfrom the supplemented at least one subsurface laser damage pattern.Following formation of this supplemental subsurface laser damage, themethod proceeds to block 538, according to which the substrate istransferred to a fracturing station.

Proceeding to block 540, in certain embodiments a carrier may be bondedto the substrate at the fracturing station to form a bonded assembly.Thereafter, according to block 542, the crystalline material isfractured along the first depth position to separate the bonded assembly(including the carrier and a removed portion of the substrate) and theremainder of the substrate, with such step serving to expose a new topsurface of the substrate. Thereafter, according to block 544, thesubstrate may be returned to the laser processing station (optionallyafter surface treatment such as grinding and/or polishing of the newlyexposed substrate surface) to enable performance of another subsurfacelaser damage step according to block 524. If the instruction setassociated with the substrate was modified to increase average laserpower according to block 532, then the modified instruction set will beused for formation of subsurface damage in performance of the stepdescribed at block 524. This modified instruction set is preferablystored in a memory and associated with the particular substrate, such asin a record of a relational database including a substrate identifierand parameters for formation of subsurface laser damage within thesubstrate.

Following fracturing of the bonded assembly from the substrate at block542, the bonded assembly may be transferred to one or more surfaceprocessing stations (according to block 546) to alter the substrateportion attached to the carrier. Examples of surface processing stepsthat may be performed include coarse grinding, edge grinding, finegrinding, and cleaning according to blocks 548, 550, 552, and 554,respectively. Thereafter, the processed substrate portion may be readyfor epitaxial growth.

Representative Computer System Useable with Systems and Methods

FIG. 54 is a schematic diagram of a generalized representation of acomputer system 600 (optionally embodied in a computing device) that canbe included in any component of the systems or methods disclosed herein.In this regard, the computer system 600 is adapted to executeinstructions from a computer-readable medium to perform these and/or anyof the functions or processing described herein. In this regard, thecomputer system 600 in FIG. 54 may include a set of instructions thatmay be executed to program and configure programmable digital signalprocessing circuits for supporting scaling of supported communicationsservices. The computer system 600 may be connected (e.g., networked) toother machines in a LAN, an intranet, an extranet, or the Internet.While only a single device is illustrated, the term “device” shall alsobe taken to include any collection of devices that individually orjointly execute a set (or multiple sets) of instructions to perform anyone or more of the methodologies discussed herein. The computer system600 may be a circuit or circuits included in an electronic board card,such as a printed circuit board (PCB), a server, a personal computer, adesktop computer, a laptop computer, a personal digital assistant (PDA),a computing pad, a mobile device, or any other device, and mayrepresent, for example, a server or a user's computer.

The computer system 600 in this embodiment includes a processing deviceor processor 602, a main memory 604 (e.g., read-only memory (ROM), flashmemory, dynamic random access memory (DRAM), such as synchronous DRAM(SDRAM), etc.), and a static memory 606 (e.g., flash memory, staticrandom access memory (SRAM), etc.), which may communicate with eachother via a data bus 608. Alternatively, the processing device 602 maybe connected to the main memory 604 and/or static memory 606 directly orvia some other connectivity means. The processing device 602 may be acontroller, and the main memory 604 or static memory 606 may be any typeof memory.

The processing device 602 represents one or more general-purposeprocessing devices, such as a microprocessor, central processing unit,or the like. More particularly, the processing device 602 may be acomplex instruction set computing (CISC) microprocessor, a reducedinstruction set computing (RISC) microprocessor, a very long instructionword (VLIW) microprocessor, a processor implementing other instructionsets, or other processors implementing a combination of instructionsets. The processing device 602 is configured to execute processinglogic in instructions for performing the operations and steps discussedherein.

The computer system 600 may further include a network interface device610. The computer system 600 also may or may not include an input 612,configured to receive input and selections to be communicated to thecomputer system 600 when executing instructions. The computer system 600also may or may not include an output 614, including but not limited toa display, a video display unit (e.g., a liquid crystal display (LCD) ora cathode ray tube (CRT)), an alphanumeric input device (e.g., akeyboard), and/or a cursor control device (e.g., a mouse).

The computer system 600 may or may not include a data storage devicethat includes instructions 616 stored in a computer readable medium 618.The instructions 616 may also reside, completely or at least partially,within the main memory 604 and/or within the processing device 602during execution thereof by the computer system 600, the main memory 604and the processing device 602 also constituting computer readablemedium. The instructions 616 may further be transmitted or received overa network 620 via the network interface device 610.

While the computer readable medium 618 is shown in an embodiment to be asingle medium, the term “computer-readable medium” should be taken toinclude a single medium or multiple media (e.g., a centralized ordistributed database, and/or associated caches and servers) that storethe one or more sets of instructions. The term “computer readablemedium” shall also be taken to include any medium that is capable ofstoring, encoding, or carrying a set of instructions for execution bythe processing device and that cause the processing device to performany one or more of the methodologies of the embodiments disclosedherein. The term “computer readable medium” shall accordingly be takento include, but not be limited to, solid-state memories, optical media,and magnetic media.

The embodiments disclosed herein include various steps. The steps of theembodiments disclosed herein may be executed or performed by hardwarecomponents or may be embodied in machine-executable instructions, whichmay be used to cause a general-purpose or special-purpose processorprogrammed with the instructions to perform the steps. Alternatively,the steps may be performed by a combination of hardware and software.

The embodiments disclosed herein may be provided as a computer programproduct, or software, that may include a machine-readable medium (orcomputer readable medium) having stored thereon instructions which maybe used to program a computer system (or other electronic devices) toperform a process according to the embodiments disclosed herein. Amachine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes: amachine-readable storage medium (e.g., ROM, random access memory(“RAM”), a magnetic disk storage medium, an optical storage medium,flash memory devices, etc.); and the like.

Unless specifically stated otherwise and as apparent from the previousdiscussion, it is appreciated that throughout the description,discussions utilizing terms such as “analyzing,” “processing,”“computing,” “determining,” “displaying,” or the like, refer to theaction and processes of a computer system, or a similar electroniccomputing device, that manipulates and transforms data and memoriesrepresented as physical (electronic) quantities within registers of thecomputer system into other data similarly represented as physicalquantities within the computer system memories or registers or othersuch information storage, transmission, or display devices.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various systems may beused with programs in accordance with the teachings herein, or it mayprove convenient to construct more specialized apparatuses to performthe required method steps. The required structure for a variety of thesesystems is disclosed in the description above. In addition, theembodiments described herein are not described with reference to anyparticular programming language. It will be appreciated that a varietyof programming languages may be used to implement the teachings of theembodiments as described herein.

Those of skill in the art will further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithms describedin connection with the embodiments disclosed herein may be implementedas electronic hardware, instructions stored in memory or in anothercomputer readable medium and executed by a processor or other processingdevice, or combinations of both. The components of the system describedherein may be employed in any circuit, hardware component, integratedcircuit (IC), or IC chip, as examples. Memory disclosed herein may beany type and size of memory and may be configured to store any type ofinformation desired. To clearly illustrate this interchangeability,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality. Howsuch functionality is implemented depends on the particular application,design choices, and/or design constraints imposed on the overall system.Skilled artisans may implement the described functionality in varyingways for each particular application, but such implementation decisionsshould not be interpreted as causing a departure from the scope of thepresent embodiments.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a processor, a Digital Signal Processor (DSP), anApplication Specific Integrated Circuit (ASIC), a Field ProgrammableGate Array (FPGA), or other programmable logic device, a discrete gateor transistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Furthermore,a controller may be a processor. A processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration).

The embodiments disclosed herein may be embodied in hardware and ininstructions that are stored in hardware, and may reside, for example,in RAM, flash memory, ROM, Electrically Programmable ROM (EPROM),Electrically Erasable Programmable ROM (EEPROM), registers, a hard disk,a removable disk, a CD-ROM, or any other form of computer readablemedium known in the art. A storage medium is coupled to the processorsuch that the processor can read information from, and write informationto, the storage medium. In the alternative, the storage medium may beintegral to the processor. The processor and the storage medium mayreside in an ASIC. The ASIC may reside in a remote station. In thealternative, the processor and the storage medium may reside as discretecomponents in a remote station, base station, or server.

It is also noted that the operational steps described in any of theembodiments herein are described to provide examples and discussion. Theoperations described may be performed in numerous different sequencesother than the illustrated sequences. Furthermore, operations describedin a single operational step may actually be performed in a number ofdifferent steps. Additionally, one or more operational steps discussedin the embodiments may be combined. Those of skill in the art will alsounderstand that information and signals may be represented using any ofa variety of technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chips,which may be referenced throughout the above description, may berepresented by voltages, currents, electromagnetic waves, magneticfields, particles, optical fields, or any combination thereof.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps, or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is in no way intendedthat any particular order be inferred.

It is contemplated that any or more features or characteristics of anyone or more embodiments disclosed herein may be combined with those ofother embodiments, unless specifically indicated to the contrary herein.

Technical benefits that may be obtained by one or more embodiments ofthe disclosure may include: enhanced reproducibility of manufacturingwafers of uniform thickness from a crystalline material substrate (e.g.,ingot) by laser processing and subsequent fracture while avoidingunnecessary material loss; addressing variation in laser powerrequirements from substrate to substrate as well as at different depthpositions in a single substrate when performing laser-assisted partingmethods; enhanced detection of uncracked regions within a crystallinematerial substrate having subsurface laser damage; reduced crystallinematerial kerf losses compared to wire sawing; reduced processing timeand increased throughput of crystalline material wafers and resultingdevices compared to wire sawing; reduced laser processing time comparedto prior laser-based methods; reduced forces required to effectuatefracture along laser damage regions; reduced need for post-separationsurface smoothing to remove laser damage following separation; and/orreduced crystalline material bowing and breakage.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A crystalline material processing methodcomprising: supplying emissions of a laser focused along a first averagedepth position within an interior of a crystalline material of asubstrate, and effecting relative lateral movement between the laser andthe substrate, to form subsurface laser damage having at least onesubsurface laser damage pattern, wherein the at least one subsurfacelaser damage pattern is configured to promote formation of at least oneplurality of cracks in the interior of the substrate propagating outwardfrom the at least one subsurface laser damage pattern; followingformation of the at least one subsurface laser damage pattern,generating at least one image of a top surface of the substrate;analyzing the at least one image to identify a condition indicative ofpresence of uncracked regions in the interior of the substrate whereinthe analyzing comprises quantifying a top area property of one or moreuncracked regions in the interior of the substrate, and comparing thetop area property to at least one predetermined threshold area property;and responsive to the analyzing, performing at least one of thefollowing steps (i) or (ii): (i) effecting relative movement between thelaser and the substrate while supplying emissions of the laser focusedwithin the interior of the substrate in at least the uncracked regionsto form supplemental subsurface laser damage to supplement the at leastone subsurface laser damage pattern and promote formation of additionalcracks in the uncracked regions along or proximate to the first averagedepth position, for formation of a first reduced thickness portion ofthe substrate; (ii) changing an instruction set, associated with thesubstrate, for forming subsurface laser damage when producing subsurfacelaser damage patterns at a second average depth position and anysubsequent average depth positions in the substrate, for formation of atleast one additional reduced thickness portion of the substrate.
 2. Thecrystalline material processing method of claim 1, wherein the analyzingcomprises quantifying a top area property of the one or more uncrackedregions in the interior of the substrate, and comparing the top areaproperty to at least one predetermined threshold area property.
 3. Thecrystalline material processing method of claim 1, wherein the at leastone predetermined threshold area property comprises a firstpredetermined threshold area property and a second predeterminedthreshold area property, wherein the second predetermined threshold areaproperty is greater than the first predetermined threshold areaproperty, and the method comprises: performing step (i) if the top areaproperty is at least as large as the first predetermined threshold areaproperty, then performing step (ii); and performing step (i) if the toparea property is at least as large as the second predetermined thresholdarea property.
 4. The crystalline material processing method of claim 1,comprising performing both of steps (i) and (ii) responsive to theanalyzing.
 5. The crystalline material processing method of claim 4,wherein the changing of the instruction set according to step (ii)comprises increasing average laser power by a value in a range of from0.15 to 0.35 watts.
 6. The crystalline material processing method ofclaim 1, wherein step (ii) comprises adjusting at least one of (a)average laser power, (b) laser focusing depth relative to an exposedsurface of the substrate, or (c) number of laser damage formationpasses, when producing subsurface laser damage patterns at the secondaverage depth position and any subsequent average depth positions in thesubstrate.
 7. The crystalline material processing method of claim 1,wherein step (i) comprises adjusting at least one of (a) average laserpower, or (b) laser focusing depth relative to an exposed surface of thesubstrate, when producing the supplemental subsurface laser damage tosupplement the at least one subsurface laser damage pattern and promoteformation of additional cracks in the uncracked regions along orproximate to the first average depth position.
 8. The crystallinematerial processing method of claim 1, wherein the substrate comprises agenerally round edge having a primary flat, and the generating of the atleast one image comprises (a) illuminating the top surface with diffuselight generated by a diffuse light source arranged to a first lateralside of the substrate and arranged substantially perpendicular to theprimary flat, and (b) capturing the at least one image with an imagingdevice arranged to an opposing second lateral side of the substrate. 9.The crystalline material processing method of claim 1, wherein: thecrystalline material comprises a hexagonal crystal structure; and thegenerating of the at least one image comprises (a) illuminating the topsurface with diffuse light generated by a diffuse light source arrangedto a first lateral side of the substrate and arranged within ±5 degreesof perpendicular to a <1120> direction of the hexagonal crystalstructure, and (b) capturing the at least one image with an imagingdevice arranged to a second lateral side of the substrate that opposesthe first lateral side.
 10. The crystalline material processing methodof claim 1, wherein: the at least one subsurface laser damage patterncomprises a first subsurface laser damage pattern and a secondsubsurface laser damage pattern that is formed after the firstsubsurface laser damage pattern; the first subsurface laser damagepattern comprises a first plurality of substantially parallel lines andthe second subsurface laser damage pattern comprises a second pluralityof substantially parallel lines; lines of the second plurality ofsubstantially parallel lines are interspersed among lines of the firstplurality of substantially parallel lines; and at least some lines ofthe second plurality of substantially parallel lines do not cross anylines of the first plurality of substantially parallel lines.
 11. Thecrystalline material processing method of claim 10, wherein each line ofthe second plurality of substantially parallel lines is arranged betweena different pair of adjacent lines of the first plurality ofsubstantially parallel lines.
 12. The crystalline material processingmethod of claim 10, wherein: the crystalline material comprises ahexagonal crystal structure; and each line of the first plurality ofsubstantially parallel lines and each line of the second plurality ofsubstantially parallel lines is within ±5 degrees of perpendicular to a<1120> direction of the hexagonal crystal structure and substantiallyparallel to a surface of the substrate.
 13. The crystalline materialprocessing method of claim 1, wherein: the at least one subsurface laserdamage pattern comprises a first subsurface laser damage pattern and asecond subsurface laser damage pattern that is formed after the firstsubsurface laser damage pattern; the at least one plurality ofsubstantially parallel lines comprises a first plurality ofsubstantially parallel lines and a second plurality of substantiallyparallel lines; lines of the first plurality of substantially parallellines are non-parallel to lines of the second plurality of substantiallyparallel lines; an angular direction of lines of the second plurality ofsubstantially parallel lines differs by no more than 10 degrees from anangular direction of lines of the first plurality of substantiallyparallel lines; and at least some lines of the second plurality ofsubstantially parallel lines do not cross any lines of the firstplurality of substantially parallel lines.
 14. The crystalline materialprocessing method of claim 13, wherein: the at least one subsurfacelaser damage pattern further comprises a third subsurface laser damagepattern that is formed after the second subsurface laser damage pattern;the at least one plurality of substantially parallel lines furthercomprises a third plurality of substantially parallel lines; the atleast one plurality of cracks comprises first, second, and thirdpluralities of cracks; the first subsurface laser damage pattern formsthe first plurality of cracks in the interior of the substratepropagating laterally outward from lines of the first plurality ofsubstantially parallel lines; the second subsurface laser damage patternforms the second plurality of cracks in the interior of the substratepropagating laterally outward from lines of the second plurality ofsubstantially parallel lines, and the second plurality of cracks isnon-connecting with the first plurality of cracks; and the thirdsubsurface laser damage pattern forms the third plurality of cracks inthe interior of the substrate propagating laterally outward from linesof the third plurality of substantially parallel lines, wherein at leastsome cracks of the third plurality of cracks connect with at least somecracks of the first plurality of cracks and with at least some cracks ofthe second plurality of cracks.
 15. The crystalline material processingmethod of claim 1, further comprising: detecting a condition indicativeof non-uniform doping of the crystalline material across at least aportion of a surface of the substrate, the non-uniform doping includinga first doping region and a second doping region; and responsive todetection of the condition indicative of non-uniform doping of thecrystalline material, performing at least one of the following steps (A)or (B): (A) altering laser power to provide laser emissions at a firstpower level when forming subsurface laser damage in the first dopingregion and provide laser emissions at a second power level when formingsubsurface laser damage in the second doping region, during formation ofthe at least one subsurface laser damage pattern; or (B) changingaverage depth for formation of subsurface laser damage in the substratewhen forming subsurface laser damage in one of the first doping regionor the second doping region.
 16. The crystalline material processingmethod of claim 1, further comprising fracturing the crystallinematerial substantially along the at least one subsurface laser damagepattern to yield first and second crystalline material portions eachhaving reduced thickness relative to the substrate, but substantially asame length and width as the substrate.
 17. The crystalline materialprocessing method of claim 1, wherein the substrate comprises siliconcarbide.
 18. The crystalline material processing method of claim 1,wherein the substrate comprises an ingot having a diameter of at least150 mm.